Chapter 3

Rodents of the superfamily Geomyoidea specialized for completely fossorial life (early Pliocene to Recent); specialized earlier (late? Oligocene and early Miocene) for semi-fossorial habits; body thickset, fusiform without apparent neck (in modern geomyids); legs short; forelegs especially stout; eyes and ears small (pinna reduced to inconspicuous crest concealed beneath pelage); tail tactile, shorter than head and body; lips closing behind incisors; cheek pouches external, fur-lined; baculum rodlike, arched, having expanded quadriform platelike base; pelage long, soft without underfur, covering body in thick coat (in some species ofOrthogeomysscant, harsh or scattered bristles); color varying from pale tints of buffy (almost white) to metallic black.Skull thick-walled, massive, angular, relatively broad, and flattened; distinctly murine form, but having zygomasseteric structure of advanced sciuromorphs, including small infraorbital canal (that transmits no part of masseter muscle) and well-developed, broad zygomatic plate; zygomata massive and widely flaring, jugals stout; rostrum robust, relatively broad and deep, and without evidence of transverse canal (as in Heteromyidae); anterior projection of nasals only slightly exceeding that of upper incisors; interorbital region usually constricted, narrower than rostrum; anterior opening of infraorbital canal far forward on side of rostrum, about half way between zygomatic plate and upper incisor and just behind premaxillary-maxillary suture, its opening countersunk in oblique sulcus (for protection from muscle contraction); postorbital process lacking, except for rudimentary knoblike projection in subgenusMacrogeomys; palate relatively narrow, its deeply sculptured surface sloping steeply downward posteriorly causing region supporting maxillary tooth-row to be markedly depressed; palatine bone reduced, forming, on two abruptly different levels, posterior margin of hard palate behind tooth-rows; parietals compressed and narrow, and most of cerebral cavity roofed by squamosals (in some species squamosals overlap lateral parts of parietals); tympanic bullae completely inferior in position and fully ossified, external meatus being developed laterally as elongated tube; mastoid not inflated, but broadly exposed at posterolateral margin of the skull; occiput large, its surface usually rugose, and paroccipital processes large and flangelike, at least in advanced groups (early Pliocene to Recent); ramus relatively short and stout, having distinctcrest and ridges for muscle attachments; coronoid process well developed, erect; articular condyle prominent; angular process prominent, reflected laterally, and in modern groups lateral extension protruding from posterior border of ramus nearly at right angle; capsule for root of lower incisor, prominent between angular process and articular condyle.Anterior surface of incisors broad and flat, always smooth on lower teeth, but either smooth or grooved on upper teeth depending on taxon; cheek teeth hypsodont, becoming progressively higher crowned in modern groups, rooted in primitive groups (late? Oligocene to middle Pliocene), rootless and ever-growing in modern groups (late Pliocene to Recent); upper and lower premolars persistently bicolumnar; upper and lower molars bicolumnar only in primitive groups (late? Oligocene and early Miocene), becoming progressively monocolumnar in advanced groups (early Pliocene to Recent), primitive bicolumnar pattern being retained on occlusal surface only in early stages of ontogeny and in third molar throughout life; enamel pattern of occlusal surface of cheek teeth based on sextituberculate prototype (see Wood and Wilson, 1936:388-391), having cusps arranged in two transverse rows of three cusps each, excepting three anterior cusps of premolars that are arranged in trefoil, especially on p4 (sometimes only one or two, rather than three, cusps develop in a particular set, especially in p4), conules absent; protostyle and endostyle in upper teeth and protostylid and hypostylid in lower teeth formed from cingulum; cusps of each row uniting with wear into transverse enamel lophs (or lophids), each tooth having two lophs, one on anterior column, protoloph and protolophid, and one on posterior column, hypoloph and hypolophid, that unite with additional wear forming continuous enamel band; enamel lacking on sides of each column in advanced lineages, thereby restricting enamel to anterior and posterior walls; with extreme reduction, posterior plates of upper teeth and, more commonly, anterior plates of lower molars, missing. Dental formula: 1/1, 0/0, 1/1, 3/3.Key to the Subfamilies of GeomyidaeA   Angular process of ramus mostly below alveolar level of mandibular tooth-row; pattern of premolar like that of molars, consisting of two subequal crests united at one or both margins of tooth; molars persistently bicolumnar; molariform teeth always rooted. Subfamily Entoptychinaep. 513A´  Angular process of ramus mostly above level of mandibular tooth-row; pattern of permolar unlike that of molars, consisting of two prisms differing in size and united at their mid-points but never at either margin; molars progressively monocolumnar, except for early Miocene forms; molariform teeth rooted only in primitive genera (late? Oligocene to middle Pliocene), and rootless and ever-growing in later genera (late Pliocene to Recent). Subfamily Geomyinaep. 514

Rodents of the superfamily Geomyoidea specialized for completely fossorial life (early Pliocene to Recent); specialized earlier (late? Oligocene and early Miocene) for semi-fossorial habits; body thickset, fusiform without apparent neck (in modern geomyids); legs short; forelegs especially stout; eyes and ears small (pinna reduced to inconspicuous crest concealed beneath pelage); tail tactile, shorter than head and body; lips closing behind incisors; cheek pouches external, fur-lined; baculum rodlike, arched, having expanded quadriform platelike base; pelage long, soft without underfur, covering body in thick coat (in some species ofOrthogeomysscant, harsh or scattered bristles); color varying from pale tints of buffy (almost white) to metallic black.

Skull thick-walled, massive, angular, relatively broad, and flattened; distinctly murine form, but having zygomasseteric structure of advanced sciuromorphs, including small infraorbital canal (that transmits no part of masseter muscle) and well-developed, broad zygomatic plate; zygomata massive and widely flaring, jugals stout; rostrum robust, relatively broad and deep, and without evidence of transverse canal (as in Heteromyidae); anterior projection of nasals only slightly exceeding that of upper incisors; interorbital region usually constricted, narrower than rostrum; anterior opening of infraorbital canal far forward on side of rostrum, about half way between zygomatic plate and upper incisor and just behind premaxillary-maxillary suture, its opening countersunk in oblique sulcus (for protection from muscle contraction); postorbital process lacking, except for rudimentary knoblike projection in subgenusMacrogeomys; palate relatively narrow, its deeply sculptured surface sloping steeply downward posteriorly causing region supporting maxillary tooth-row to be markedly depressed; palatine bone reduced, forming, on two abruptly different levels, posterior margin of hard palate behind tooth-rows; parietals compressed and narrow, and most of cerebral cavity roofed by squamosals (in some species squamosals overlap lateral parts of parietals); tympanic bullae completely inferior in position and fully ossified, external meatus being developed laterally as elongated tube; mastoid not inflated, but broadly exposed at posterolateral margin of the skull; occiput large, its surface usually rugose, and paroccipital processes large and flangelike, at least in advanced groups (early Pliocene to Recent); ramus relatively short and stout, having distinctcrest and ridges for muscle attachments; coronoid process well developed, erect; articular condyle prominent; angular process prominent, reflected laterally, and in modern groups lateral extension protruding from posterior border of ramus nearly at right angle; capsule for root of lower incisor, prominent between angular process and articular condyle.

Anterior surface of incisors broad and flat, always smooth on lower teeth, but either smooth or grooved on upper teeth depending on taxon; cheek teeth hypsodont, becoming progressively higher crowned in modern groups, rooted in primitive groups (late? Oligocene to middle Pliocene), rootless and ever-growing in modern groups (late Pliocene to Recent); upper and lower premolars persistently bicolumnar; upper and lower molars bicolumnar only in primitive groups (late? Oligocene and early Miocene), becoming progressively monocolumnar in advanced groups (early Pliocene to Recent), primitive bicolumnar pattern being retained on occlusal surface only in early stages of ontogeny and in third molar throughout life; enamel pattern of occlusal surface of cheek teeth based on sextituberculate prototype (see Wood and Wilson, 1936:388-391), having cusps arranged in two transverse rows of three cusps each, excepting three anterior cusps of premolars that are arranged in trefoil, especially on p4 (sometimes only one or two, rather than three, cusps develop in a particular set, especially in p4), conules absent; protostyle and endostyle in upper teeth and protostylid and hypostylid in lower teeth formed from cingulum; cusps of each row uniting with wear into transverse enamel lophs (or lophids), each tooth having two lophs, one on anterior column, protoloph and protolophid, and one on posterior column, hypoloph and hypolophid, that unite with additional wear forming continuous enamel band; enamel lacking on sides of each column in advanced lineages, thereby restricting enamel to anterior and posterior walls; with extreme reduction, posterior plates of upper teeth and, more commonly, anterior plates of lower molars, missing. Dental formula: 1/1, 0/0, 1/1, 3/3.

Key to the Subfamilies of Geomyidae

A   Angular process of ramus mostly below alveolar level of mandibular tooth-row; pattern of premolar like that of molars, consisting of two subequal crests united at one or both margins of tooth; molars persistently bicolumnar; molariform teeth always rooted. Subfamily Entoptychinae

p. 513

A´  Angular process of ramus mostly above level of mandibular tooth-row; pattern of permolar unlike that of molars, consisting of two prisms differing in size and united at their mid-points but never at either margin; molars progressively monocolumnar, except for early Miocene forms; molariform teeth rooted only in primitive genera (late? Oligocene to middle Pliocene), and rootless and ever-growing in later genera (late Pliocene to Recent). Subfamily Geomyinae

p. 514

SubfamilyEntoptychinaeMiller and Gidley, 1918

Anterior face of upper incisor usually smooth, sometimes bearing faint groove in center or near medial margin of tooth, at least inGregorymys; cheek teeth hypsodont, medium to high crowned, and rooted in all butEntoptychus(has rootless, ever-growing teeth); cheek teeth identical in form, premolars resembling molars and lower cheek teeth mirror images of upper teeth; crowns biprismatic, having two columns joined at edge of protomeres (for description of term, see discussion of primitive morphotype onp. 537) and with persistent lateral fissure between them; lateral re-entrant fold deep, penetrating at least half width of crown, from external side in upper teeth and internal side in lower teeth (in specialized genusEntoptychuslophs, upon additional wear, join also at edge of parameres, thus uniting columns at both ends and thereby enclosing interior part of lateral fissure as a transverse fossette in center of tooth); enamel investment of prisms usually complete, including inflection bordering re-entrant folds, occlusal pattern becoming interrupted with wear only inEntoptychus, where enamel disappears first from sides of crowns (followingunion of anterior and posterior columns at both sides) and later, in final stages of attrition, from anterior wall of lower molars and posterior wall of upper molars.Maxillary bone without pronounced vertical depth in part supporting cheek teeth, its inferior border only slightly lower than inferior border of premaxillary and alveolar lips of molariform teeth consequently approximately level with, or slightly below, alveolar lip of upper incisor; squamosal without lateral expansion, therefore, meatal tube of auditory bulla separated from zygomatic process of squamosal by deep, well-developed postglenoid notch; angular part of mandible below alveolar level of mandibular cheek teeth; angular process only slightly reflected laterally; coronoid process low, tip only slightly above condyle.For information concerning the structure and relationships of the known genera, and for accounts of species, see Wood (1936). A list of the named genera in order of specialization is as follows:*PleurolicusCope, 1878. Proc. Amer. Phil. Soc., 18:66.*GregorymysWood, 1936. Amer. Mus. Novit., 866:9.*GrangerimusWood, 1936. Amer. Mus. Novit., 866:13.*EntoptychusCope, 1878. Proc. Amer. Phil. Soc., 18:64.Five new species have been described since Wood's (1936) revision. They are:Pleurolicus clasoniMacDonald (1963:180);Gregorymys kayiWood (1950:335);Gregorymys montanensisHibbard and Keenmon (1950:198);Grangerimus dakotensisMacDonald (1963:182);Grangerimus sellardsiHibbard and Wilson (1950:623).

Anterior face of upper incisor usually smooth, sometimes bearing faint groove in center or near medial margin of tooth, at least inGregorymys; cheek teeth hypsodont, medium to high crowned, and rooted in all butEntoptychus(has rootless, ever-growing teeth); cheek teeth identical in form, premolars resembling molars and lower cheek teeth mirror images of upper teeth; crowns biprismatic, having two columns joined at edge of protomeres (for description of term, see discussion of primitive morphotype onp. 537) and with persistent lateral fissure between them; lateral re-entrant fold deep, penetrating at least half width of crown, from external side in upper teeth and internal side in lower teeth (in specialized genusEntoptychuslophs, upon additional wear, join also at edge of parameres, thus uniting columns at both ends and thereby enclosing interior part of lateral fissure as a transverse fossette in center of tooth); enamel investment of prisms usually complete, including inflection bordering re-entrant folds, occlusal pattern becoming interrupted with wear only inEntoptychus, where enamel disappears first from sides of crowns (followingunion of anterior and posterior columns at both sides) and later, in final stages of attrition, from anterior wall of lower molars and posterior wall of upper molars.

Maxillary bone without pronounced vertical depth in part supporting cheek teeth, its inferior border only slightly lower than inferior border of premaxillary and alveolar lips of molariform teeth consequently approximately level with, or slightly below, alveolar lip of upper incisor; squamosal without lateral expansion, therefore, meatal tube of auditory bulla separated from zygomatic process of squamosal by deep, well-developed postglenoid notch; angular part of mandible below alveolar level of mandibular cheek teeth; angular process only slightly reflected laterally; coronoid process low, tip only slightly above condyle.

For information concerning the structure and relationships of the known genera, and for accounts of species, see Wood (1936). A list of the named genera in order of specialization is as follows:

*PleurolicusCope, 1878. Proc. Amer. Phil. Soc., 18:66.

*GregorymysWood, 1936. Amer. Mus. Novit., 866:9.

*GrangerimusWood, 1936. Amer. Mus. Novit., 866:13.

*EntoptychusCope, 1878. Proc. Amer. Phil. Soc., 18:64.

Five new species have been described since Wood's (1936) revision. They are:Pleurolicus clasoniMacDonald (1963:180);Gregorymys kayiWood (1950:335);Gregorymys montanensisHibbard and Keenmon (1950:198);Grangerimus dakotensisMacDonald (1963:182);Grangerimus sellardsiHibbard and Wilson (1950:623).

SubfamilyGeomyinaeBaird, 1858

Anterior face of upper incisor primitively smooth, grooves consistently developed only in one modern lineage (Geomyini); cheek teeth hypsodont, primitively rooted and having crown of medium height (late Oligocene to middle Pliocene), being higher crowned, rootless and ever-growing in modern lineages (late Pliocene to Recent); primitively crowns of cheek teeth biprismatic, having two columns joined at mid-points by narrow isthmus and entire crown sheathed in continuous band of enamel; premolars retaining primitive biprismatic form, anterior and posterior columns never uniting at edge of protomeres or parameres, and with both lateral re-entrant folds persistent throughout life; primitive biprismatic pattern becoming decidedly modified in molars (except in M3), having two prisms progressively uniting into one column by reduction and loss of lateral inflections, primitive biprismatic patterns being retained only in early stages of ontogeny; third upper molars retaining, at least partially, primitive bicolumnar pattern (except in Thomomyini), with relatively broad isthmus and horizontally shallow re-entrant folds, lingual fold sometimes wanting; enamel pattern becoming discontinuous (late Pliocene to Recent) owing to loss of enamel from sides of each column; remaining enamel restricted to anterior and posterior plates, or cutting blades, and enamel bordering lateral inflections in premolars (considering both sides together, these plates constitute essentially two transverse cutting blades); enamel pattern of M3 varying, depending on taxon; with specialization, anterior plates of lower molars and posterior plates of upper premolar and molars may be reduced or lost; except in primitive species (early Miocene), no enamel fossettes retained in adult dentitions.Maxillary bone having pronounced vertical depth in part supporting cheek teeth, inferior border arching downward well below inferior border of premaxillary; consequently, alveolar lips of molariform teeth decidedly below level of alveolar lip of upper incisor; squamosal with marked lateral expansion at expense of postglenoid notch; notch compressed and reduced between meatal tube of auditory bulla and zygomatic process of squamosal; angular part of mandible mostly above alveolar level of mandibular cheek teeth; angular process reflected laterally at right angles to axis of ramus and developed into heavy knoblike projection; coronoid process well developed, tip decidedly higher thancondyle; fossorial specializations remarkably well developed in advanced lineages, degree of specialization of primitive Miocene species unknown but probably only semi-fossorial as in Entoptychinae.Key to the Tribes of the GeomyinaeA   Enamel investment complete and uninterrupted, even in final (adult) stages of wear; cheek teeth rooted, with crowns of medium height; third lower molar biprismatic, the two columns separated by inner and outer re-entrant folds as in lower premolar. Tribe Dikkomyinip. 515A´  Enamel investment incomplete and discontinuous, reduced, at least in final (adult) stages of wear, to interrupted enamel plates; cheek teeth rootless and ever-growing (except in extinct genusPliogeomys), crowns of maximum height; third lower molar monoprismatic, without trace of inner and outer re-entrant folds as in first and second lower molars.B   Upper incisors smooth, occasionally with a fine indistinct groove near inner margin of tooth; form of third upper molar same as M1 and M2, monoprismatic, anteroposteriorly compressed, and having transverse enamel plates on both anterior and posterior faces, and without suggestion of either labial or lingual re-entrant folds; basitemporal fossa absent (except for a shallow depression in one Recent species,T. townsendii); forefoot small and narrow with claws not elongated for digging. Tribe Thomomyinip. 518B´  Upper incisors grooved, bearing either one or two sulci; form of third upper molar distinctly different from M1 and M2, fully or partially biprismatic (with a few exceptions discussed beyond), without marked anteroposterior compression (either subtriangular, elongated, suborbicular or quadriform in cross-section, but not elliptical as in M1 and M2), and having typical transverse anterior plate and two lateral plates (varying in their development, depending on taxa), but no posterior plate, and with lateral re-entrant folds usually developed, especially labial inflection (although sometimes minute in a few species, as described beyond); basitemporal fossa well-developed, although occasionally shallow or absent (primitive species ofZygogeomys); forefoot large and broad, with elongated claws for digging. Tribe Geomyinip. 521

Anterior face of upper incisor primitively smooth, grooves consistently developed only in one modern lineage (Geomyini); cheek teeth hypsodont, primitively rooted and having crown of medium height (late Oligocene to middle Pliocene), being higher crowned, rootless and ever-growing in modern lineages (late Pliocene to Recent); primitively crowns of cheek teeth biprismatic, having two columns joined at mid-points by narrow isthmus and entire crown sheathed in continuous band of enamel; premolars retaining primitive biprismatic form, anterior and posterior columns never uniting at edge of protomeres or parameres, and with both lateral re-entrant folds persistent throughout life; primitive biprismatic pattern becoming decidedly modified in molars (except in M3), having two prisms progressively uniting into one column by reduction and loss of lateral inflections, primitive biprismatic patterns being retained only in early stages of ontogeny; third upper molars retaining, at least partially, primitive bicolumnar pattern (except in Thomomyini), with relatively broad isthmus and horizontally shallow re-entrant folds, lingual fold sometimes wanting; enamel pattern becoming discontinuous (late Pliocene to Recent) owing to loss of enamel from sides of each column; remaining enamel restricted to anterior and posterior plates, or cutting blades, and enamel bordering lateral inflections in premolars (considering both sides together, these plates constitute essentially two transverse cutting blades); enamel pattern of M3 varying, depending on taxon; with specialization, anterior plates of lower molars and posterior plates of upper premolar and molars may be reduced or lost; except in primitive species (early Miocene), no enamel fossettes retained in adult dentitions.

Maxillary bone having pronounced vertical depth in part supporting cheek teeth, inferior border arching downward well below inferior border of premaxillary; consequently, alveolar lips of molariform teeth decidedly below level of alveolar lip of upper incisor; squamosal with marked lateral expansion at expense of postglenoid notch; notch compressed and reduced between meatal tube of auditory bulla and zygomatic process of squamosal; angular part of mandible mostly above alveolar level of mandibular cheek teeth; angular process reflected laterally at right angles to axis of ramus and developed into heavy knoblike projection; coronoid process well developed, tip decidedly higher thancondyle; fossorial specializations remarkably well developed in advanced lineages, degree of specialization of primitive Miocene species unknown but probably only semi-fossorial as in Entoptychinae.

Key to the Tribes of the Geomyinae

A   Enamel investment complete and uninterrupted, even in final (adult) stages of wear; cheek teeth rooted, with crowns of medium height; third lower molar biprismatic, the two columns separated by inner and outer re-entrant folds as in lower premolar. Tribe Dikkomyini

p. 515

A´  Enamel investment incomplete and discontinuous, reduced, at least in final (adult) stages of wear, to interrupted enamel plates; cheek teeth rootless and ever-growing (except in extinct genusPliogeomys), crowns of maximum height; third lower molar monoprismatic, without trace of inner and outer re-entrant folds as in first and second lower molars.

B   Upper incisors smooth, occasionally with a fine indistinct groove near inner margin of tooth; form of third upper molar same as M1 and M2, monoprismatic, anteroposteriorly compressed, and having transverse enamel plates on both anterior and posterior faces, and without suggestion of either labial or lingual re-entrant folds; basitemporal fossa absent (except for a shallow depression in one Recent species,T. townsendii); forefoot small and narrow with claws not elongated for digging. Tribe Thomomyini

p. 518

B´  Upper incisors grooved, bearing either one or two sulci; form of third upper molar distinctly different from M1 and M2, fully or partially biprismatic (with a few exceptions discussed beyond), without marked anteroposterior compression (either subtriangular, elongated, suborbicular or quadriform in cross-section, but not elliptical as in M1 and M2), and having typical transverse anterior plate and two lateral plates (varying in their development, depending on taxa), but no posterior plate, and with lateral re-entrant folds usually developed, especially labial inflection (although sometimes minute in a few species, as described beyond); basitemporal fossa well-developed, although occasionally shallow or absent (primitive species ofZygogeomys); forefoot large and broad, with elongated claws for digging. Tribe Geomyini

p. 521

TribeDikkomyini, new tribe

Genotype.—DikkomysWood, 1936.Chronologic and geographic range.—Early to Middle Pliocene (early Arikareean to mid-Hemphillian) in western United States. Known from Miocene fossil sites in Montana, South Dakota, and Nebraska and Pliocene sites in South Dakota, Oregon, Nevada, and southern California. For precise localities see accounts ofDikkomysandPliosaccomysbeyond.Diagnosis.—Small Geomyinae; lacking specializations of more advanced tribes; upper incisors smooth, at least inPliosaccomys; molariform teeth always rooted and having crowns of medium height; enamel investment of cheek teeth complete and uninterrupted in all stages of wear; crowns of molars primitively biprismatic, having two columns united at mid-points, thus forming narrow isthmus separating lateral re-entrant folds as in premolars, and, with wear, also uniting secondarily at protomeres (with exception of third lower molars), consequently, isolating remnant of that inflection as shallow fossette (columns uniting first at protomeres inPliosaccomys); anterior and posterior columns of first and second molars, both above and below, becoming progressively united into one column in advanced Dikkomyini (early and middle Pliocene), but m3 (M3 unknown) retaining primitive biprismatic pattern, with columns joined at centers but never at protomeres (for details of dentition see generic accounts);mandible stout, its angle mostly above mandibular tooth-row; masseteric ridge low; basitemporal fossa barely discernable in some fragments ofPliosaccomys; postcranial skeleton unknown.Key to the Genera of the Tribe DikkomyiniA   Molars biprismatic throughout life; anterior and posterior lophs of first and second molars in pre-final stages of wear uniting first at their mid-points and later at edge of protomeres; anterior lophid of lower premolar having distinct anteroexternal inflection. GenusDikkomysp. 516A'  First and second molars becoming monoprismatic in final (adult?) stages of wear, biprismatic only in pre-final stages of wear; third molars persistently biprismatic; anterior and posterior lophs of first and second molars uniting first at edge of protomeres; anterior lophid of lower premolar lacking anteroexternal inflection. GenusPliosaccomysp. 517

Genotype.—DikkomysWood, 1936.

Chronologic and geographic range.—Early to Middle Pliocene (early Arikareean to mid-Hemphillian) in western United States. Known from Miocene fossil sites in Montana, South Dakota, and Nebraska and Pliocene sites in South Dakota, Oregon, Nevada, and southern California. For precise localities see accounts ofDikkomysandPliosaccomysbeyond.

Diagnosis.—Small Geomyinae; lacking specializations of more advanced tribes; upper incisors smooth, at least inPliosaccomys; molariform teeth always rooted and having crowns of medium height; enamel investment of cheek teeth complete and uninterrupted in all stages of wear; crowns of molars primitively biprismatic, having two columns united at mid-points, thus forming narrow isthmus separating lateral re-entrant folds as in premolars, and, with wear, also uniting secondarily at protomeres (with exception of third lower molars), consequently, isolating remnant of that inflection as shallow fossette (columns uniting first at protomeres inPliosaccomys); anterior and posterior columns of first and second molars, both above and below, becoming progressively united into one column in advanced Dikkomyini (early and middle Pliocene), but m3 (M3 unknown) retaining primitive biprismatic pattern, with columns joined at centers but never at protomeres (for details of dentition see generic accounts);mandible stout, its angle mostly above mandibular tooth-row; masseteric ridge low; basitemporal fossa barely discernable in some fragments ofPliosaccomys; postcranial skeleton unknown.

Key to the Genera of the Tribe Dikkomyini

A   Molars biprismatic throughout life; anterior and posterior lophs of first and second molars in pre-final stages of wear uniting first at their mid-points and later at edge of protomeres; anterior lophid of lower premolar having distinct anteroexternal inflection. GenusDikkomys

p. 516

A'  First and second molars becoming monoprismatic in final (adult?) stages of wear, biprismatic only in pre-final stages of wear; third molars persistently biprismatic; anterior and posterior lophs of first and second molars uniting first at edge of protomeres; anterior lophid of lower premolar lacking anteroexternal inflection. GenusPliosaccomys

p. 517

GenusDikkomysWood

1936.DikkomysWood, Amer. Mus. Novit., 866:26, July 2.Type.—Dikkomys matthewiWood, 1936, from Lower Harrison deposits near Agate, Sioux County, Nebraska.Chronologic range.—Early Miocene, from early Arikareean (Lower Harrison local fauna of Nebraska) to middle Miocene, late Hemingfordian (Upper Rosebud local fauna, South Dakota, and the Deep River Formation, Montana). According to MacDonald (1963:149-150), the Upper Rosebud is middle Miocene rather than early Miocene.Description.—Size small, about as in small kinds ofThomomys; known only from fragmentary mandible, including molariform dentition in place, and isolated cheek teeth, including M1 (see Wood, 1936:26-28 and fig. 32; Galbreath, 1948:316-317 and fig. 1; and Black, 1961:13-14 and fig. 58); upper incisors unknown; cheek teeth hyposodont, persistently rooted, and having crowns of medium height compared with Recent geomyids; enamel investment complete and uninterrupted in all molariform teeth in all stages of wear; P4 unknown, but probably formed like p4; p4 persistently biprismatic, two crowns joined at mid-points by relatively narrow isthmus separating lateral re-entrant folds; anterior lophid of p4 having distinct anteroexternal inflection; molars also biprismatic throughout life; two lophids of lower molars first uniting at mid-points as in p4, and, with additional wear, m1 and m2 secondarily uniting at edge of protomeres and forming isolated enamel fossette between point of connection (detailed description of stages of wear discussed in account of phylogeny of subfamily); m3 permanently joined at mid-point only, without lateral union at edge of protomeres; upper molars, judging by M1 (M2 and M3 unknown), having same pattern as lower molars, but first union of lophs decidedly on lingual side of center, consequently, lingual re-entrant fold small; M1 probably developing U-pattern in advanced stages of wear by union of protomeres, with minute lingual fossette developing in transition as lophs secondarily become united at lingual edge of columns; mandible stout and geomyidlike; masseteric ridge weakly developed; basitemporal fossa absent.Evidently,Dikkomys matthewiis more primitive thanDikkomys woodi. The modified H-pattern in m1 and m2, with the metalophid and hypolophid joined at both their mid-points and also at their protomeres (by union of the protostylid and hypostylid in the lower dentition), is persistent throughout life. Therefore, the enclosed enamel fossette is not eradicated with wear. In m1 and m2 ofDikkomys woodi, the fossette is shallower, and, at least in advanced stages of wear, it would disappear, therefore, forming a U-pattern on the occlusal surface, as in M1 and M2, but lateral inflection horizontally shallow rather than deep as in entoptychines.Specimen (No. P 26284 FMNH) reported asDikkomys matthewiby Galbreath (1948:316) is referable to the recently described speciesDikkomys woodiBlack, 1961.Specimens examined.—One, no. P 26284, Field Mus. Nat. Hist., from upper Rosebud, Shannon Co., South Dakota.Referred species.—two:Dikkomys matthewiWood, 1936. Amer. Mus. Novit., 866:26, July. Type from early Arikareean Lower Harrison deposits (early Miocene) near Agate, Sioux County, Nebraska.Dikkomys woodiBlack, 1961. Postilla, Yale Peabody Museum, 48:13, January 16. Type from Deep River Formation, late Hemingfordian (middle Miocene), Meagher County, Montana; also known from Upper Rosebud deposits (middle Miocene) near Wounded Knee, Shannon County, South Dakota.

1936.DikkomysWood, Amer. Mus. Novit., 866:26, July 2.

Type.—Dikkomys matthewiWood, 1936, from Lower Harrison deposits near Agate, Sioux County, Nebraska.

Chronologic range.—Early Miocene, from early Arikareean (Lower Harrison local fauna of Nebraska) to middle Miocene, late Hemingfordian (Upper Rosebud local fauna, South Dakota, and the Deep River Formation, Montana). According to MacDonald (1963:149-150), the Upper Rosebud is middle Miocene rather than early Miocene.

Description.—Size small, about as in small kinds ofThomomys; known only from fragmentary mandible, including molariform dentition in place, and isolated cheek teeth, including M1 (see Wood, 1936:26-28 and fig. 32; Galbreath, 1948:316-317 and fig. 1; and Black, 1961:13-14 and fig. 58); upper incisors unknown; cheek teeth hyposodont, persistently rooted, and having crowns of medium height compared with Recent geomyids; enamel investment complete and uninterrupted in all molariform teeth in all stages of wear; P4 unknown, but probably formed like p4; p4 persistently biprismatic, two crowns joined at mid-points by relatively narrow isthmus separating lateral re-entrant folds; anterior lophid of p4 having distinct anteroexternal inflection; molars also biprismatic throughout life; two lophids of lower molars first uniting at mid-points as in p4, and, with additional wear, m1 and m2 secondarily uniting at edge of protomeres and forming isolated enamel fossette between point of connection (detailed description of stages of wear discussed in account of phylogeny of subfamily); m3 permanently joined at mid-point only, without lateral union at edge of protomeres; upper molars, judging by M1 (M2 and M3 unknown), having same pattern as lower molars, but first union of lophs decidedly on lingual side of center, consequently, lingual re-entrant fold small; M1 probably developing U-pattern in advanced stages of wear by union of protomeres, with minute lingual fossette developing in transition as lophs secondarily become united at lingual edge of columns; mandible stout and geomyidlike; masseteric ridge weakly developed; basitemporal fossa absent.

Evidently,Dikkomys matthewiis more primitive thanDikkomys woodi. The modified H-pattern in m1 and m2, with the metalophid and hypolophid joined at both their mid-points and also at their protomeres (by union of the protostylid and hypostylid in the lower dentition), is persistent throughout life. Therefore, the enclosed enamel fossette is not eradicated with wear. In m1 and m2 ofDikkomys woodi, the fossette is shallower, and, at least in advanced stages of wear, it would disappear, therefore, forming a U-pattern on the occlusal surface, as in M1 and M2, but lateral inflection horizontally shallow rather than deep as in entoptychines.

Specimen (No. P 26284 FMNH) reported asDikkomys matthewiby Galbreath (1948:316) is referable to the recently described speciesDikkomys woodiBlack, 1961.

Specimens examined.—One, no. P 26284, Field Mus. Nat. Hist., from upper Rosebud, Shannon Co., South Dakota.

Referred species.—two:

Dikkomys matthewiWood, 1936. Amer. Mus. Novit., 866:26, July. Type from early Arikareean Lower Harrison deposits (early Miocene) near Agate, Sioux County, Nebraska.

Dikkomys woodiBlack, 1961. Postilla, Yale Peabody Museum, 48:13, January 16. Type from Deep River Formation, late Hemingfordian (middle Miocene), Meagher County, Montana; also known from Upper Rosebud deposits (middle Miocene) near Wounded Knee, Shannon County, South Dakota.

GenusPliosaccomysWilson

1936.PliosaccomysWilson, Carnegie Inst. Washington Publ., 473:20, May 21.Type.—Pliosaccomys dubiusWilson, 1936, from Smiths Valley local fauna in Lyon County, Nevada.Chronologic range.—Early Pliocene, late Clarendonian (Wolf Creek local fauna, South Dakota, and Nettle Springs local fauna, California) to Middle Pliocene, middle part of Hemphillian (Smiths Valley local fauna, Nevada, and McKay Reservoir and Otis Basin local faunas, Oregon).Description.—Size small (alveolar length of mandibular tooth-row measuring 6.0 in holotype), about as inThomomys monticola; upper incisor relatively broad and flat, having anterior face smooth, without trace of grooving; crowns of cheek teeth of medium height and rooted; enamel investment continuous and uninterrupted in all stages of wear; premolars permanently, biprismatic; P4 having anterior prism subtriangular and decidedly smaller that sub-crescentic posterior prism, and joined near centers by narrow, obliquely oriented isthmus; p4 having anterior prism subovate, posterior prism strongly compressed anteroposteriorly, and joined at mid-points by relatively broad and straight isthmus; first and second molars, both above and below, monoprismatic in final (?adult) stage of wear, derived ontogenetically from primitive bilophate pattern by coalescence of two columns into one; M1 and M2 mirror images of m1 and m2 in pre-final stages of wear, two columns first uniting at edge of protomeres forming U-pattern, and primitive H-pattern never developing in either series (for detailed description of stages of wear, see account of phylogeny,p. 546); m3 (M3 unknown, but probably with same form as in Geomyini, seep. 552) persistently biprismatic, two columns joined by relatively broad isthmus at centers, consequently, forming H-pattern of primitive ancestors; rostrum heavy and broad as in modern geomyids; palate narrow and strongly ribbed; mandible stout; masseteric ridge and fossa well developed; basitemporal fossa absent.Specimens examined.—Six, nos. 1796 (holotype)—1799, 1804 and 1806 (CIT) now in the Los Angeles County Museum, all from Smiths Valley local fauna, Middle Pliocene, Nevada.Referred species.—two:*Pliosaccomys dubiusWilson, 1936. Carnegie Inst. Washington Publ., 743:20, May 21. Known from early and middle Pliocene faunas including Wolf Creek local fauna (late Clarendonian), Shannon County, South Dakota; McKay Reservoir local fauna and Otis Basin local fauna (Hemphillian), Oregon; type from Smiths Valley local fauna (probably middle Hemphillian), Lyon County, Nevada.*Pliosaccomys wilsoniJames, 1963. Univ. California Publ. Geol. Sci., 45:101, June 26. Type from Nettle Springs local fauna of late Clarendonian (early Pliocene), Ventura County, California.

1936.PliosaccomysWilson, Carnegie Inst. Washington Publ., 473:20, May 21.

Type.—Pliosaccomys dubiusWilson, 1936, from Smiths Valley local fauna in Lyon County, Nevada.

Chronologic range.—Early Pliocene, late Clarendonian (Wolf Creek local fauna, South Dakota, and Nettle Springs local fauna, California) to Middle Pliocene, middle part of Hemphillian (Smiths Valley local fauna, Nevada, and McKay Reservoir and Otis Basin local faunas, Oregon).

Description.—Size small (alveolar length of mandibular tooth-row measuring 6.0 in holotype), about as inThomomys monticola; upper incisor relatively broad and flat, having anterior face smooth, without trace of grooving; crowns of cheek teeth of medium height and rooted; enamel investment continuous and uninterrupted in all stages of wear; premolars permanently, biprismatic; P4 having anterior prism subtriangular and decidedly smaller that sub-crescentic posterior prism, and joined near centers by narrow, obliquely oriented isthmus; p4 having anterior prism subovate, posterior prism strongly compressed anteroposteriorly, and joined at mid-points by relatively broad and straight isthmus; first and second molars, both above and below, monoprismatic in final (?adult) stage of wear, derived ontogenetically from primitive bilophate pattern by coalescence of two columns into one; M1 and M2 mirror images of m1 and m2 in pre-final stages of wear, two columns first uniting at edge of protomeres forming U-pattern, and primitive H-pattern never developing in either series (for detailed description of stages of wear, see account of phylogeny,p. 546); m3 (M3 unknown, but probably with same form as in Geomyini, seep. 552) persistently biprismatic, two columns joined by relatively broad isthmus at centers, consequently, forming H-pattern of primitive ancestors; rostrum heavy and broad as in modern geomyids; palate narrow and strongly ribbed; mandible stout; masseteric ridge and fossa well developed; basitemporal fossa absent.

Specimens examined.—Six, nos. 1796 (holotype)—1799, 1804 and 1806 (CIT) now in the Los Angeles County Museum, all from Smiths Valley local fauna, Middle Pliocene, Nevada.

Referred species.—two:

*Pliosaccomys dubiusWilson, 1936. Carnegie Inst. Washington Publ., 743:20, May 21. Known from early and middle Pliocene faunas including Wolf Creek local fauna (late Clarendonian), Shannon County, South Dakota; McKay Reservoir local fauna and Otis Basin local fauna (Hemphillian), Oregon; type from Smiths Valley local fauna (probably middle Hemphillian), Lyon County, Nevada.

*Pliosaccomys wilsoniJames, 1963. Univ. California Publ. Geol. Sci., 45:101, June 26. Type from Nettle Springs local fauna of late Clarendonian (early Pliocene), Ventura County, California.

TribeThomomyini, new tribe

Type.—ThomomysWied-Neuwied, 1839.Chronologic and geographic range.—Known from late Pliocene (early Blancan) to Recent. Known primarily from western North America from southern Canada south to Central México in Pliocene, Pleistocene and Recent and in middle and late Pleistocene of Maryland and Florida.Diagnosis.—Size small to medium (basilar length exclusive ofT. bulbivorus, measuring from approximately 24 to 45, including both males and females); upper incisors without grooving, excepting fine, indistinct sulcus rarely near inner margin (grooving more common inT. monticolathan in other Recent species); crowns of cheek teeth high, rooted and ever-growing; all molars, including M3, monoprismatic and anteroposteriorly compressed, sometimes (especially in subadults) having slight inflection on labial side in upper teeth and lingual side in lower teeth; molars bicolumnar in pre-final stages of wear (seen in juvenal teeth only), patterns of wear in both upper and lower molars resembling those ofPliosaccomys, except that crowns of m3 and M3 unite into single column in final stages of wear; enamel pattern interrupted in all cheek teeth, loss occurring only at sides of each column; transverse enamel blade completely covering posterior face of both P4 and p4; all upper and lower molars with two transverse enamel blades, one on anterior surface and one on posterior surface, of each tooth, including M3; small third plate sometimes persistent on broad side of tooth, labial side in upper molars and lingual side in lower molars (T. bulbivorus); skull generalized, neither unusually narrow and deep or broad and flat; usually without marked cresting or rugosity; masseteric ridge well developed and massive; basitemporal fossa absent, sometimes shallow depression forming inT. townsendii; pelage soft, never harsh or hispid, covering body with thick coat of hair; forefoot exceptionally small for fossorial mammal, claws not especially long; body form remarkably fossorial.The tribe Thomomyini is monotypic, including only the genusThomomys.

Type.—ThomomysWied-Neuwied, 1839.

Chronologic and geographic range.—Known from late Pliocene (early Blancan) to Recent. Known primarily from western North America from southern Canada south to Central México in Pliocene, Pleistocene and Recent and in middle and late Pleistocene of Maryland and Florida.

Diagnosis.—Size small to medium (basilar length exclusive ofT. bulbivorus, measuring from approximately 24 to 45, including both males and females); upper incisors without grooving, excepting fine, indistinct sulcus rarely near inner margin (grooving more common inT. monticolathan in other Recent species); crowns of cheek teeth high, rooted and ever-growing; all molars, including M3, monoprismatic and anteroposteriorly compressed, sometimes (especially in subadults) having slight inflection on labial side in upper teeth and lingual side in lower teeth; molars bicolumnar in pre-final stages of wear (seen in juvenal teeth only), patterns of wear in both upper and lower molars resembling those ofPliosaccomys, except that crowns of m3 and M3 unite into single column in final stages of wear; enamel pattern interrupted in all cheek teeth, loss occurring only at sides of each column; transverse enamel blade completely covering posterior face of both P4 and p4; all upper and lower molars with two transverse enamel blades, one on anterior surface and one on posterior surface, of each tooth, including M3; small third plate sometimes persistent on broad side of tooth, labial side in upper molars and lingual side in lower molars (T. bulbivorus); skull generalized, neither unusually narrow and deep or broad and flat; usually without marked cresting or rugosity; masseteric ridge well developed and massive; basitemporal fossa absent, sometimes shallow depression forming inT. townsendii; pelage soft, never harsh or hispid, covering body with thick coat of hair; forefoot exceptionally small for fossorial mammal, claws not especially long; body form remarkably fossorial.

The tribe Thomomyini is monotypic, including only the genusThomomys.

GenusThomomysWied-Neuwied

1839.ThomomysWied-Neuwied, Nova Acta Phys. Med. Acad. Caesar. Leop.-Carol., 19(1):377.1836.OryctomysEydoux and Gervais (in part), Mag. de Zool., 6:20, pl. 21. Type:Oryctomys(Saccophorus)bottae, from coast of California, probably near Monterey.1903.MegascapheusElliot, Field Columb. Mus., Publ. 76, Zool. Ser., 3(11):190, July 25. Type:Diplostoma bulbivorumRichardson, from Columbia River, probably near Portland, Ore.1933.PleisothomomysGidley and Gazin, Jour. Mamm. 14:354. Type:Pleisothomomys potomacensisGidley and Gazin, from Pleistocene, Cumberland Cave local fauna, Allegany County, Maryland.Chronologic range.—Known from late Pliocene to Recent.Description.—Same as that given for the tribe Thomomyini above.Discussion.—Features characterizingThomomysand the tribe Thomomyini are more advanced than those characterizing the tribe Dikkomyini. Also, theThomomyini retain more of the primitive features of the Geomyinae than do the more specialized tribe Geomyini.Specializations are few, but include the third molar being a single column both above and below, enamel plates, and a masseteric ridge.Key to the Subgenera ofThomomysA   Molars sub-crescent or ovate in cross-section, not becoming abruptly narrower at one end of tooth. SubgenusPleisothomomysp. 519A´  Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. SubgenusThomomysp. 520

1839.ThomomysWied-Neuwied, Nova Acta Phys. Med. Acad. Caesar. Leop.-Carol., 19(1):377.

1836.OryctomysEydoux and Gervais (in part), Mag. de Zool., 6:20, pl. 21. Type:Oryctomys(Saccophorus)bottae, from coast of California, probably near Monterey.

1903.MegascapheusElliot, Field Columb. Mus., Publ. 76, Zool. Ser., 3(11):190, July 25. Type:Diplostoma bulbivorumRichardson, from Columbia River, probably near Portland, Ore.

1933.PleisothomomysGidley and Gazin, Jour. Mamm. 14:354. Type:Pleisothomomys potomacensisGidley and Gazin, from Pleistocene, Cumberland Cave local fauna, Allegany County, Maryland.

Chronologic range.—Known from late Pliocene to Recent.

Description.—Same as that given for the tribe Thomomyini above.

Discussion.—Features characterizingThomomysand the tribe Thomomyini are more advanced than those characterizing the tribe Dikkomyini. Also, theThomomyini retain more of the primitive features of the Geomyinae than do the more specialized tribe Geomyini.

Specializations are few, but include the third molar being a single column both above and below, enamel plates, and a masseteric ridge.

Key to the Subgenera ofThomomys

A   Molars sub-crescent or ovate in cross-section, not becoming abruptly narrower at one end of tooth. SubgenusPleisothomomys

p. 519

A´  Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. SubgenusThomomys

p. 520

SubgenusPleisothomomysGidley and Gazin

1933.PleisothomomysGidley and Gazin, Jour. Mamm., 14:354, November 13.Type.—Pleisothomomys potomacensisGidley and Gazin, 1933.Chronologic range.—Late Pliocene (Hagerman local fauna, Idaho) to late Pleistocene. The latest records are from the fauna of Saber-tooth Cave, Florida, a late Pleistocene assemblage that probably was deposited in the Sangamon. The middle and late Pleistocene records are from the eastern United States, suggesting that the subgenusPleisothomomyswas restricted to that region while the subgenusThomomysoccupied the western United States and parts of Canada and México as it does today.Description and Comparison.—Separated from subgenusThomomysonly on basis of sub-crescentic shaped molars (only jaw fragments and isolated teeth known), seemingly a primitive feature of the genus. This dental structure continued into the late Pleistocene; none of the Recent species expresses this feature of the molars, although the molars ofThomomys vetusof the late Pleistocene (Wisconsin deposits), referred to the subgenusThomomyson the basis of its alleged relationship toThomomys townsendii(see Davis, 1937:156-158), are less distinctly pear-shaped, and are more sub-crescentic, than in any other known species of the subgenusThomomys.PleisothomomysGidley and Gazin (loc. cit.) was proposed as a genus but is here considered as of no more than subgeneric worth, and is recognized because of the apparent constancy of the sub-crescentic molars in the earlier members of the genus and in those populations ofThomomysoccurring in Pleistocene times in the eastern United States.Referred species.—Three (all extinct):*Thomomys gidleyiWilson, 1933. Carnegie Inst. Washington Publ. 440:122, December. Type from Hagerman beds, late Pliocene, Idaho.*Thomomys potomacensisGidley and Gazin, 1933. Jour. Mamm., 14:354, November 13. Type from Cumberland Cave, middle and late Pleistocene, Maryland.*Thomomys orientalisSimpson, 1928. Amer. Mus. Novit., 328:6, October 26. Type from Saber-tooth Cave, late Pleistocene, Florida.

1933.PleisothomomysGidley and Gazin, Jour. Mamm., 14:354, November 13.

Type.—Pleisothomomys potomacensisGidley and Gazin, 1933.

Chronologic range.—Late Pliocene (Hagerman local fauna, Idaho) to late Pleistocene. The latest records are from the fauna of Saber-tooth Cave, Florida, a late Pleistocene assemblage that probably was deposited in the Sangamon. The middle and late Pleistocene records are from the eastern United States, suggesting that the subgenusPleisothomomyswas restricted to that region while the subgenusThomomysoccupied the western United States and parts of Canada and México as it does today.

Description and Comparison.—Separated from subgenusThomomysonly on basis of sub-crescentic shaped molars (only jaw fragments and isolated teeth known), seemingly a primitive feature of the genus. This dental structure continued into the late Pleistocene; none of the Recent species expresses this feature of the molars, although the molars ofThomomys vetusof the late Pleistocene (Wisconsin deposits), referred to the subgenusThomomyson the basis of its alleged relationship toThomomys townsendii(see Davis, 1937:156-158), are less distinctly pear-shaped, and are more sub-crescentic, than in any other known species of the subgenusThomomys.PleisothomomysGidley and Gazin (loc. cit.) was proposed as a genus but is here considered as of no more than subgeneric worth, and is recognized because of the apparent constancy of the sub-crescentic molars in the earlier members of the genus and in those populations ofThomomysoccurring in Pleistocene times in the eastern United States.

Referred species.—Three (all extinct):

*Thomomys gidleyiWilson, 1933. Carnegie Inst. Washington Publ. 440:122, December. Type from Hagerman beds, late Pliocene, Idaho.

*Thomomys potomacensisGidley and Gazin, 1933. Jour. Mamm., 14:354, November 13. Type from Cumberland Cave, middle and late Pleistocene, Maryland.

*Thomomys orientalisSimpson, 1928. Amer. Mus. Novit., 328:6, October 26. Type from Saber-tooth Cave, late Pleistocene, Florida.

SubgenusThomomysWied-Neuwied

1839.ThomomysWied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar. Leop. Carol., 19(1):377.1903.MegascapheusElliot, Field Columb. Mus., Publ. 76, Zool. Ser., 3 (11):190, July 25. Type:Diplostoma bulbivorumRichardson, from Columbia River, probably near Portland, Oregon.Type.—Thomomys rufescensWied-Neuwied, 1839.Chronologic range.—Early Pleistocene (Broadwater-Lisco local fauna, Nebraska) to Recent. Numerous records, mostly isolated teeth, from nearly all stratigraphic levels of the Pleistocene (for details, see account of fossil record).Description.—Molars pear-shaped in cross-section, becoming abruptly narrow at one end of the tooth. The teeth of the late Pleistocene speciesThomomys vetusare less distinctly pear-shaped than other referred species (see remarks in the description of the subgenusPleisothomomys).Essentially on the basis of its significantly larger size and details of the skull, Elliott (1903:190) proposed subgeneric recognition ofThomomys bulbivorusand described the subgenusMegascapheusto include it. Also the molars ofThomomys bulbivorususually have a small enamel plate, both above and below, bordering the persistent inflection on the protomere end of the tooth; each lateral plate is isolated from the transverse plates on the anterior and posterior walls of the tooth. In my opinion these features do not warrant subgeneric recognition; however, these characters do distinctly separateThomomys bulbivorusfrom other groups of species, and the character of the molars suggests retention of a primitive trait. Therefore, I propose that the unique structure of this species be recognized by setting it apart in thebulbivorusspecies-group.Referred species.—Ten species, three extinct, placed in three species-groups (the numerous subspecies of this genus are listed in Miller and Kellogg, 1955:276-332, and Hall and Kelson, 1959:412-447).bulbivorusspecies-groupThomomys bulbivorus(Richardson, 1829). Fauna Boreali-Americana, 1:206. Type from Columbia River, probably near Portland, Oregon.umbrinusspecies-group*Thomomys scudderiHay, 1921. Proc. U. S. Nat. Mus., 49:614. Type from Fossil Lake beds, late Pleistocene, Oregon.Thomomys umbrinus(Richardson, 1829). Fauna Boreali-Americana, 1:202. Type from southern México, probably near Boca de Monte, Veracruz.Thomomys bottae(Eydoux and Gervais, 1836). Mag. de Zool., Paris, 6:23. Type from coast of California, probably near Monterey.*Thomomys vetusDavis, 1937. Jour. Mamm., 18:156, May 12. Type from Fossil Lake beds, late Pleistocene, Oregon.Thomomys townsendii(Bachman, 1839). Jour. Acad. Nat. Sci. Philadelphia, 8:105. Type probably from near Nampa, Canyon Co., Idaho (erroneously given as "Columbia River").talpoidesspecies-group*Thomomys microdonSinclair, 1905. Bull. Dept. Geol. Univ. California, 4:145-161. Type from Potter Creek Cave, late Pleistocene, California.Thomomys monticolaJ. A. Allen, 1893. Bull. Amer. Mus. Nat. Hist., 5:48, April 28. Type from Mt. Tallac, 7500 ft., El Dorado Co., California.Thomomys talpoides(Richardson, 1828). Zool. Jour., 3:518. Type locality fixed at near Fort Carlton (Carlton House), Saskatchewan River, Saskatchewan, Canada.Thomomys mazamaMerriam, 1897. Proc. Biol. Soc. Washington, 11:214, July 15. Type from Anna Creek, 6000 ft., near Crater Lake, Mt. Mazama, Klamath Co., Washington.

1839.ThomomysWied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar. Leop. Carol., 19(1):377.

1903.MegascapheusElliot, Field Columb. Mus., Publ. 76, Zool. Ser., 3 (11):190, July 25. Type:Diplostoma bulbivorumRichardson, from Columbia River, probably near Portland, Oregon.

Type.—Thomomys rufescensWied-Neuwied, 1839.

Chronologic range.—Early Pleistocene (Broadwater-Lisco local fauna, Nebraska) to Recent. Numerous records, mostly isolated teeth, from nearly all stratigraphic levels of the Pleistocene (for details, see account of fossil record).

Description.—Molars pear-shaped in cross-section, becoming abruptly narrow at one end of the tooth. The teeth of the late Pleistocene speciesThomomys vetusare less distinctly pear-shaped than other referred species (see remarks in the description of the subgenusPleisothomomys).

Essentially on the basis of its significantly larger size and details of the skull, Elliott (1903:190) proposed subgeneric recognition ofThomomys bulbivorusand described the subgenusMegascapheusto include it. Also the molars ofThomomys bulbivorususually have a small enamel plate, both above and below, bordering the persistent inflection on the protomere end of the tooth; each lateral plate is isolated from the transverse plates on the anterior and posterior walls of the tooth. In my opinion these features do not warrant subgeneric recognition; however, these characters do distinctly separateThomomys bulbivorusfrom other groups of species, and the character of the molars suggests retention of a primitive trait. Therefore, I propose that the unique structure of this species be recognized by setting it apart in thebulbivorusspecies-group.

Referred species.—Ten species, three extinct, placed in three species-groups (the numerous subspecies of this genus are listed in Miller and Kellogg, 1955:276-332, and Hall and Kelson, 1959:412-447).

bulbivorusspecies-group

Thomomys bulbivorus(Richardson, 1829). Fauna Boreali-Americana, 1:206. Type from Columbia River, probably near Portland, Oregon.

umbrinusspecies-group

*Thomomys scudderiHay, 1921. Proc. U. S. Nat. Mus., 49:614. Type from Fossil Lake beds, late Pleistocene, Oregon.

Thomomys umbrinus(Richardson, 1829). Fauna Boreali-Americana, 1:202. Type from southern México, probably near Boca de Monte, Veracruz.

Thomomys bottae(Eydoux and Gervais, 1836). Mag. de Zool., Paris, 6:23. Type from coast of California, probably near Monterey.

*Thomomys vetusDavis, 1937. Jour. Mamm., 18:156, May 12. Type from Fossil Lake beds, late Pleistocene, Oregon.

Thomomys townsendii(Bachman, 1839). Jour. Acad. Nat. Sci. Philadelphia, 8:105. Type probably from near Nampa, Canyon Co., Idaho (erroneously given as "Columbia River").

talpoidesspecies-group

*Thomomys microdonSinclair, 1905. Bull. Dept. Geol. Univ. California, 4:145-161. Type from Potter Creek Cave, late Pleistocene, California.

Thomomys monticolaJ. A. Allen, 1893. Bull. Amer. Mus. Nat. Hist., 5:48, April 28. Type from Mt. Tallac, 7500 ft., El Dorado Co., California.

Thomomys talpoides(Richardson, 1828). Zool. Jour., 3:518. Type locality fixed at near Fort Carlton (Carlton House), Saskatchewan River, Saskatchewan, Canada.

Thomomys mazamaMerriam, 1897. Proc. Biol. Soc. Washington, 11:214, July 15. Type from Anna Creek, 6000 ft., near Crater Lake, Mt. Mazama, Klamath Co., Washington.

TribeGeomyini, new tribe

Genotype.—GeomysRafinesque, 1817.Chronologic and geographic range.—Known from late middle Pliocene deposits to Recent. The range of living members extends from extreme southern Manitoba and the southeastern United States south to southern Panamá, and probably northern Colombia, South America.Diagnosis.—Size small to large (condylobasal length of skull 33.0 to 73.0 in adults, including both sexes); sexual dimorphism marked, sometimes strongly, females being smaller than males, especially in cranial dimensions; upper incisors invariably grooved, number and position of grooves varying according to genus; cheek teeth high-crowned and ever-growing, except in one primitive genus (Pliogeomys); all three lower molars and M1 and M2 monoprismatic, and elliptical in cross-section in final stages of wear (teeth of young, subadult, and adult animals); primitive biprismatic patterns (as known from Recent specimens) occurring only in pre-final stages of wear (teeth of juveniles only); biprismatic patterns of lower molars as inDikkomys, and upper molars as inPliosaccomys(for detailed description of these patterns, see account beyond of the phylogeny of the Geomyinae); m3 becoming monoprismatic, anteroposteriorly compressed and elliptical in cross-section like m1 and m2, but M3 remaining, with rare exceptions (see accounts ofGeomysandPappogeomysbeyond), at least partially biprismatic throughout life, having one or both lateral inflections usually persisting (with exceptions) and developing various occlusal shapes (subtriangular, elongate, obcordate, suborbiculate, or quadriform) but never elliptical.Enamel of cheek teeth reduced to interrupted plates, with exception of p4 inPliogeomys; plate on posterior wall of P4 variable, occurring completely across posterior surface in primitive members, but progressively reduced to lingual side only or completely lost in modern genera (see generic accounts beyond for detailed description); both anterior and posterior plates usually retained in M1 and M2, posterior plate sometimes reduced to lingual side or completely lost (as inPappogeomys) but anterior plate always completely retained; M3 usually having three plates, one anterior and two lateral; posterior plate wanting (sometimes lingual plate moved to posterior position); plates retained completely across posterior walls of all lower cheek teeth with no reduction, but anterior plates of m1-3 always lacking, except in primitive genusPliogeomys(only Geomyini having both anterior and posterior enamel plates on lower molars).Skull primitively generalized, but becoming specialized towards either dolichocephaly (Orthogeomys) or platycephaly (Pappogeomys) in two modern genera; skull highly specialized for fossorial life; mandible stout and deep, angular process being high and diverging laterally at right angles to ramus; masseteric ridge and fossa weakly developed in primitive members, becoming well developed and massive in modern genera; basitemporal fossa absent in primitive forms (Pliogeomysand early members ofZygogeomys); pelage usually soft, but harsh and hispid in some genera; forefeet broad and massive, claws long and stout for digging; body form remarkably fossorial.The tribe Geomyini includes the most highly specialized members of the subfamily Geomyinae.Key to the Genera of the Tribe GeomyiniA   Cheek teeth rooted; p4 with uninterrupted enamel loop; enamel plates on both anterior and posterior walls of m1 and m2; masseteric ridge weakly developed, low, not massive. GenusPliogeomysp. 522A´  Cheek teeth rootless, ever-growing; p4 with enamel investment interrupted at ends of columns, consequently, forming four isloted plates; enamel plate retained only on posterior wall of m1 and m2, anterior wall without trace of enamel (except rarely in pre-final stage of wear inGeomys tobinensisof middle Pleistocene); masseteric crest strongly developed and massive.B   Enamel plate on posterior wall of P4, but usually restricted to lingual end of tooth (usually absent in subgenusOrthogeomysof genusOrthogeomys); M3 conspicuously bicolumnar, longer than wide owing to elongation of posterior loph.C   Upper incisor bisulcate; skull generalized; rostrum relatively narrow; length of labial enamel plate of M3 decidedly less than length of lingual plate; pelage soft and thick. GenusZygogeomysp. 523C´  Upper incisor unisulcate; skull strongly dolichocephalic; rostrum remarkably broad and massive; length of lingual plate of M3 approximately equal to, or greater than, length of labial plate; pelage harsh, often hispid and scant. GenusOrthogeomysp. 528B´  Posterior wall of P4 without trace of enamel; M3 not strongly bicolumnar, having shallow re-entrant fold on labial side, and crown no longer than wide owing to shortness of posterior loph.D   Upper incisor bisulcate; skull generalized; both anterior and posterior walls of M1 and M2 having complete enamel plates. GenusGeomysp. 525D´  Upper incisor unisulcate; skull generalized or tending towards platycephaly; enamel plate on posterior wall of M1 usually reduced to lingual side or absent (complete only in one species,Pappogeomys bulleri); enamel plate on posterior wall of M2 also absent in advanced species (subgenusCratogeomys). GenusPappogeomysp. 532

Genotype.—GeomysRafinesque, 1817.

Chronologic and geographic range.—Known from late middle Pliocene deposits to Recent. The range of living members extends from extreme southern Manitoba and the southeastern United States south to southern Panamá, and probably northern Colombia, South America.

Diagnosis.—Size small to large (condylobasal length of skull 33.0 to 73.0 in adults, including both sexes); sexual dimorphism marked, sometimes strongly, females being smaller than males, especially in cranial dimensions; upper incisors invariably grooved, number and position of grooves varying according to genus; cheek teeth high-crowned and ever-growing, except in one primitive genus (Pliogeomys); all three lower molars and M1 and M2 monoprismatic, and elliptical in cross-section in final stages of wear (teeth of young, subadult, and adult animals); primitive biprismatic patterns (as known from Recent specimens) occurring only in pre-final stages of wear (teeth of juveniles only); biprismatic patterns of lower molars as inDikkomys, and upper molars as inPliosaccomys(for detailed description of these patterns, see account beyond of the phylogeny of the Geomyinae); m3 becoming monoprismatic, anteroposteriorly compressed and elliptical in cross-section like m1 and m2, but M3 remaining, with rare exceptions (see accounts ofGeomysandPappogeomysbeyond), at least partially biprismatic throughout life, having one or both lateral inflections usually persisting (with exceptions) and developing various occlusal shapes (subtriangular, elongate, obcordate, suborbiculate, or quadriform) but never elliptical.

Enamel of cheek teeth reduced to interrupted plates, with exception of p4 inPliogeomys; plate on posterior wall of P4 variable, occurring completely across posterior surface in primitive members, but progressively reduced to lingual side only or completely lost in modern genera (see generic accounts beyond for detailed description); both anterior and posterior plates usually retained in M1 and M2, posterior plate sometimes reduced to lingual side or completely lost (as inPappogeomys) but anterior plate always completely retained; M3 usually having three plates, one anterior and two lateral; posterior plate wanting (sometimes lingual plate moved to posterior position); plates retained completely across posterior walls of all lower cheek teeth with no reduction, but anterior plates of m1-3 always lacking, except in primitive genusPliogeomys(only Geomyini having both anterior and posterior enamel plates on lower molars).

Skull primitively generalized, but becoming specialized towards either dolichocephaly (Orthogeomys) or platycephaly (Pappogeomys) in two modern genera; skull highly specialized for fossorial life; mandible stout and deep, angular process being high and diverging laterally at right angles to ramus; masseteric ridge and fossa weakly developed in primitive members, becoming well developed and massive in modern genera; basitemporal fossa absent in primitive forms (Pliogeomysand early members ofZygogeomys); pelage usually soft, but harsh and hispid in some genera; forefeet broad and massive, claws long and stout for digging; body form remarkably fossorial.

The tribe Geomyini includes the most highly specialized members of the subfamily Geomyinae.

Key to the Genera of the Tribe Geomyini

A   Cheek teeth rooted; p4 with uninterrupted enamel loop; enamel plates on both anterior and posterior walls of m1 and m2; masseteric ridge weakly developed, low, not massive. GenusPliogeomys

p. 522

A´  Cheek teeth rootless, ever-growing; p4 with enamel investment interrupted at ends of columns, consequently, forming four isloted plates; enamel plate retained only on posterior wall of m1 and m2, anterior wall without trace of enamel (except rarely in pre-final stage of wear inGeomys tobinensisof middle Pleistocene); masseteric crest strongly developed and massive.

B   Enamel plate on posterior wall of P4, but usually restricted to lingual end of tooth (usually absent in subgenusOrthogeomysof genusOrthogeomys); M3 conspicuously bicolumnar, longer than wide owing to elongation of posterior loph.

C   Upper incisor bisulcate; skull generalized; rostrum relatively narrow; length of labial enamel plate of M3 decidedly less than length of lingual plate; pelage soft and thick. GenusZygogeomys

p. 523

C´  Upper incisor unisulcate; skull strongly dolichocephalic; rostrum remarkably broad and massive; length of lingual plate of M3 approximately equal to, or greater than, length of labial plate; pelage harsh, often hispid and scant. GenusOrthogeomys

p. 528

B´  Posterior wall of P4 without trace of enamel; M3 not strongly bicolumnar, having shallow re-entrant fold on labial side, and crown no longer than wide owing to shortness of posterior loph.

D   Upper incisor bisulcate; skull generalized; both anterior and posterior walls of M1 and M2 having complete enamel plates. GenusGeomys

p. 525

D´  Upper incisor unisulcate; skull generalized or tending towards platycephaly; enamel plate on posterior wall of M1 usually reduced to lingual side or absent (complete only in one species,Pappogeomys bulleri); enamel plate on posterior wall of M2 also absent in advanced species (subgenusCratogeomys). GenusPappogeomys

p. 532

GenusPliogeomysHibbard

1954.PliogeomysHibbard, Michigan Acad. Sci., Arts and Letters, 39:353.Genotype.—Pliogeomys buisiHibbard, 1954, from Buis Ranch local fauna (middle Pliocene), Beaver County, Oklahoma.Chronologic range.—Latest Middle Pliocene, known only from the highest part of the Hemphillian mammalian fauna (Buis Ranch local fauna, Oklahoma). Professor Hibbard informs me (personal communication) that he found the type, a right ramus, lying on the surface near the base of the fossil beds. The isolated teeth of small geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392) may also be referable to this genus. The Saw Rock Canyon local fauna may also be middle Pliocene in age but is considered to be from the later part of the late Pliocene, and, therefore, somewhat younger than the Buis Ranch local fauna (Hibbard,op. cit.:342).Description and discussion.—The size of members of this small genus of the Geomyinae is about the same as in smaller adults ofGeomys bursarius. According to Hibbard (op. cit.:353), the holotype is smaller than specimens from the Rexroad local fauna referred toGeomys quinniand larger than specimens referred toZygogeomyscf.minor. The cheek teeth are rooted, and the crowns are as high as those of living geomyids. The upper incisor is bisulcate, and the inner groove is fine and indistinct in places.Of the molariform dentition only the lower premolar and first two lower molars are known. The enamel investment of p4 is complete, and would not be subject to interruption at any stage of wear; the two prisms are joined at their mid-points, and the isthmus of dentine is relatively broad (as inPliosaccomys) when compared with modern pocket gophers of this tribe. Also, the re-entrant folds, rather than having parallel sides, diverge broadly to the sides. The divergence is especially noticeable in the labial fold. The lower deciduous premolar would have formed essentially the same enamel pattern with wear as observed inNerterogeomys[=Zygogeomys] cf.minor(see Hibbard, 1954:fig. 5, A and B) andPliosaccomys dubius(see Wilson, 1936; pl. 1, fig. 1). Each molar is a single column in the final stages of wear; pre-final stages are unknown. Anterior and posterior enamel plates are present on m1 and m2 (m3 has not been recovered). The dentine tracts of m1 are exposed over a relatively wide surface; therefore, the enamel plates are distinctly separated. The tracts of dentine of m2 are much narrower than in m1 and the enamel plates are barely separated at the anterolateral margin of the tooth. Possibly the enamel band of m2 was continuous in an earlier stage of wear.The mandible is stout and its general construction not unlike that in modern geomyines. The capsule at the base of the angular process that receives the terminal end of the lower incisor is well developed. The base of the angular processes is preserved, and suggests that the process was short and decidedly smaller than in living examples of the tribe. The masseteric ridge is distinct but weakly developed, and not at all massive as in living pocket gophers. The mental foramen is immediately anterior, and slightly ventral, to the anterior extension of the crest. The basitemporal fossa is absent as such, but its position is marked by a slight depression.Specimens examined.—Two rami; nos. 29147 (holotype) and 33446; several isolated teeth 30194 and 30195, including an upper incisor and a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo.Referred species.—One.*Pliogeomys buisiHibbard, 1954. Papers Michigan Acad. Sci., Arts, and Letters, 39:353. Type from Buis local fauna, latest middle Pliocene, Beaver County, Oklahoma.

1954.PliogeomysHibbard, Michigan Acad. Sci., Arts and Letters, 39:353.

Genotype.—Pliogeomys buisiHibbard, 1954, from Buis Ranch local fauna (middle Pliocene), Beaver County, Oklahoma.

Chronologic range.—Latest Middle Pliocene, known only from the highest part of the Hemphillian mammalian fauna (Buis Ranch local fauna, Oklahoma). Professor Hibbard informs me (personal communication) that he found the type, a right ramus, lying on the surface near the base of the fossil beds. The isolated teeth of small geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392) may also be referable to this genus. The Saw Rock Canyon local fauna may also be middle Pliocene in age but is considered to be from the later part of the late Pliocene, and, therefore, somewhat younger than the Buis Ranch local fauna (Hibbard,op. cit.:342).

Description and discussion.—The size of members of this small genus of the Geomyinae is about the same as in smaller adults ofGeomys bursarius. According to Hibbard (op. cit.:353), the holotype is smaller than specimens from the Rexroad local fauna referred toGeomys quinniand larger than specimens referred toZygogeomyscf.minor. The cheek teeth are rooted, and the crowns are as high as those of living geomyids. The upper incisor is bisulcate, and the inner groove is fine and indistinct in places.

Of the molariform dentition only the lower premolar and first two lower molars are known. The enamel investment of p4 is complete, and would not be subject to interruption at any stage of wear; the two prisms are joined at their mid-points, and the isthmus of dentine is relatively broad (as inPliosaccomys) when compared with modern pocket gophers of this tribe. Also, the re-entrant folds, rather than having parallel sides, diverge broadly to the sides. The divergence is especially noticeable in the labial fold. The lower deciduous premolar would have formed essentially the same enamel pattern with wear as observed inNerterogeomys[=Zygogeomys] cf.minor(see Hibbard, 1954:fig. 5, A and B) andPliosaccomys dubius(see Wilson, 1936; pl. 1, fig. 1). Each molar is a single column in the final stages of wear; pre-final stages are unknown. Anterior and posterior enamel plates are present on m1 and m2 (m3 has not been recovered). The dentine tracts of m1 are exposed over a relatively wide surface; therefore, the enamel plates are distinctly separated. The tracts of dentine of m2 are much narrower than in m1 and the enamel plates are barely separated at the anterolateral margin of the tooth. Possibly the enamel band of m2 was continuous in an earlier stage of wear.

The mandible is stout and its general construction not unlike that in modern geomyines. The capsule at the base of the angular process that receives the terminal end of the lower incisor is well developed. The base of the angular processes is preserved, and suggests that the process was short and decidedly smaller than in living examples of the tribe. The masseteric ridge is distinct but weakly developed, and not at all massive as in living pocket gophers. The mental foramen is immediately anterior, and slightly ventral, to the anterior extension of the crest. The basitemporal fossa is absent as such, but its position is marked by a slight depression.

Specimens examined.—Two rami; nos. 29147 (holotype) and 33446; several isolated teeth 30194 and 30195, including an upper incisor and a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo.

Referred species.—One.

*Pliogeomys buisiHibbard, 1954. Papers Michigan Acad. Sci., Arts, and Letters, 39:353. Type from Buis local fauna, latest middle Pliocene, Beaver County, Oklahoma.

GenusZygogeomysMerriam

1895.ZygogeomysMerriam, N. Amer. Fauna, 8:195, January 31.1942.NerterogeomysGazin, Proc. U. S. Nat. Mus., 92:507 (type,Geomys persimilisHay, 1927).Type.—Zygogeomys trichopusMerriam, 1895, from Nahuatzen, Michoacán.Chronologic range.—Late Pliocene (Benson and Curtis Ranch local faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent.Description and discussion.—The size is small to medium for the subfamily Geomyinae. This genus is distinguished principally by the retention of primitive features. In the living species, the skull is generalized, rather than specialized toward either extreme dolichocephaly or platycephaly. The angular process is short, barely exceeding the lateral extensions of the mastoid process of the squamosal. The rostrum is remarkably narrow in relation to its length. The jugal is reduced and displaced ventrally, causing the maxillary arm of the zygomata to articulate with the squamosal arm of the zygomata along the dorsal border of the zygomatic arch (a feature observed also inOrthogeomys cherriei costaricensis).The upper incisor, recovered in material from the late Pliocene and middle Pleistocene, is bisulcate as in the genusGeomysand the primitive genusPliogeomys. The enamel plate across the posterior wall of P4 is either complete (late Pliocene to late Pleistocene) or restricted to the lingual half of the tooth (always restricted in living species). The Pliocene specimens of the Rexroad local fauna referred toNerterogeomyscf.minorby Hibbard (1950:138-139) are exceptional. In these specimens the length and position of the posterior enamel plate is variable; however, all but one specimen had persistant enamel. Evidently, in approximately 43 per cent of the specimens, a complete enamel blade was present (see Paulson, 1961:139), and in the others (except the one without any enamel) the plate was restricted to a small area of the ventral surface, usually on the lingual side of the loph. Hibbard suggested that the decrease in size of the plate, and its restriction to the lingual side, may be a function of age. Hence, most adults would be characterized by the reduced posterior plate on the upper premolar. Although age may be the important factor, intragroup variation cannot be ruled out. It is of interest to note that in all specimens from the Benson (type series ofP. minor) and Curtis Ranch local faunas, the former of late Pliocene age and the latter of middle Pleistocene age, the enamel plates are complete on the posterior face of the upper premolar. As mentioned before, the specimens from Kansas may actually represent the transitional stages of the early evolution ofGeomysin which the posterior plate of P4 is entirely lost. The enamel pattern of p4 is like that in other members of the tribe (excepting the genusPliogeomys). The re-entrant angles of P4 and p4 are widely open (obtuse) in the examples recovered from late Pliocene and middle Pleistocene deposits, representing retention of a trait that is primitive in the Geomyini (see account of phylogeny).M1 and M2 are elliptical in cross-section and each has an enamel plate on both the anterior and posterior surface. In the living species (Z. trichopus), the posterior enamel plate fails to reach the labial margin of the tooth and is restricted to the lingual two-thirds of the posterior surface; however, the enamel plates are complete in the late Pliocene species (Z. minor) and the middle Pleistocene species (Z. persimilis), being only slightly separated from the anterior plate by narrow tracts of dentine on the ends of the tooth. M3 is partly biprismatic in the living species, the two incompletely divided lophs being separated by a distinct outer sulcus. The posterior loph is elongated and forms a conspicuous heel paralleling the evolution of this trait in the genusOrthogeomys; therefore, the crown is longer than wide. The posterior part of the tooth is protected by two lateral enamel plates; of the two, the lingual plate is especially long and extends to the end of the heel. M3 has not been recovered in the Pliocene species, but in the middle Pleistocene species (Z. persimilis) M3 is subtriangular, no longer than wide, and the lateral inflections are weakly developed. The trend towards elongation of M3 evidently occurred in late Pleistocene evolution of the genus. All three of the inferior molars are elliptical, and only the posterior enamel plate is present (as in all other genera of the tribe exceptPliogeomys).The masseteric ridge of the mandible is well developed. In the late Pliocene speciesZ. persimilisandZ. minorthe mental foramen is directly beneath the anterior extension of the masseteric ridge, but in the living species,Z. trichopus, the foramen lies well anterior to the ridge. The basitemporal fossa in the livingspecies is well developed and deep; in the Pliocene species it is usually distinct but shallow (late Pliocene specimens ofZ. minor).Referred species.—Three (two extinct and one living; the last has two subspecies):*Zygogeomys minor(Gidley), 1922. U. S. Geol. Surv. Prof. Paper, 131:123, December 26. Type from Benson local fauna (late Pliocene), Cochise County, Arizona; also known from the Rexroad local fauna, Meade County, Kansas.*Zygogeomys persimilisHay, 1927. Carnegie Inst. Washington Publ., 136. Originally described by Gidley, 1922 (U. S. Geol. Surv. Prof. Papers, 131:123, December 26) asGeomys parvidenswhich was preoccupied byG. parvidensBrown, 1908. Type from Curtis Ranch local fauna (middle Pleistocene), Cochise County, Arizona.Zygogeomys trichopus trichopusMerriam, 1895. N. Amer. Fauna, 8:196, January 31. Type from Nahuatzen, Michoacán.Zygogeomys trichopus tarascensisGoldman, 1938. Proc. Biol. Soc. Washington, 51:211, December 23. Type from 6 mi. SE Pátzcuaro, 8,000 ft., Michoacán.

1895.ZygogeomysMerriam, N. Amer. Fauna, 8:195, January 31.

1942.NerterogeomysGazin, Proc. U. S. Nat. Mus., 92:507 (type,Geomys persimilisHay, 1927).

Type.—Zygogeomys trichopusMerriam, 1895, from Nahuatzen, Michoacán.

Chronologic range.—Late Pliocene (Benson and Curtis Ranch local faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent.

Description and discussion.—The size is small to medium for the subfamily Geomyinae. This genus is distinguished principally by the retention of primitive features. In the living species, the skull is generalized, rather than specialized toward either extreme dolichocephaly or platycephaly. The angular process is short, barely exceeding the lateral extensions of the mastoid process of the squamosal. The rostrum is remarkably narrow in relation to its length. The jugal is reduced and displaced ventrally, causing the maxillary arm of the zygomata to articulate with the squamosal arm of the zygomata along the dorsal border of the zygomatic arch (a feature observed also inOrthogeomys cherriei costaricensis).

The upper incisor, recovered in material from the late Pliocene and middle Pleistocene, is bisulcate as in the genusGeomysand the primitive genusPliogeomys. The enamel plate across the posterior wall of P4 is either complete (late Pliocene to late Pleistocene) or restricted to the lingual half of the tooth (always restricted in living species). The Pliocene specimens of the Rexroad local fauna referred toNerterogeomyscf.minorby Hibbard (1950:138-139) are exceptional. In these specimens the length and position of the posterior enamel plate is variable; however, all but one specimen had persistant enamel. Evidently, in approximately 43 per cent of the specimens, a complete enamel blade was present (see Paulson, 1961:139), and in the others (except the one without any enamel) the plate was restricted to a small area of the ventral surface, usually on the lingual side of the loph. Hibbard suggested that the decrease in size of the plate, and its restriction to the lingual side, may be a function of age. Hence, most adults would be characterized by the reduced posterior plate on the upper premolar. Although age may be the important factor, intragroup variation cannot be ruled out. It is of interest to note that in all specimens from the Benson (type series ofP. minor) and Curtis Ranch local faunas, the former of late Pliocene age and the latter of middle Pleistocene age, the enamel plates are complete on the posterior face of the upper premolar. As mentioned before, the specimens from Kansas may actually represent the transitional stages of the early evolution ofGeomysin which the posterior plate of P4 is entirely lost. The enamel pattern of p4 is like that in other members of the tribe (excepting the genusPliogeomys). The re-entrant angles of P4 and p4 are widely open (obtuse) in the examples recovered from late Pliocene and middle Pleistocene deposits, representing retention of a trait that is primitive in the Geomyini (see account of phylogeny).

M1 and M2 are elliptical in cross-section and each has an enamel plate on both the anterior and posterior surface. In the living species (Z. trichopus), the posterior enamel plate fails to reach the labial margin of the tooth and is restricted to the lingual two-thirds of the posterior surface; however, the enamel plates are complete in the late Pliocene species (Z. minor) and the middle Pleistocene species (Z. persimilis), being only slightly separated from the anterior plate by narrow tracts of dentine on the ends of the tooth. M3 is partly biprismatic in the living species, the two incompletely divided lophs being separated by a distinct outer sulcus. The posterior loph is elongated and forms a conspicuous heel paralleling the evolution of this trait in the genusOrthogeomys; therefore, the crown is longer than wide. The posterior part of the tooth is protected by two lateral enamel plates; of the two, the lingual plate is especially long and extends to the end of the heel. M3 has not been recovered in the Pliocene species, but in the middle Pleistocene species (Z. persimilis) M3 is subtriangular, no longer than wide, and the lateral inflections are weakly developed. The trend towards elongation of M3 evidently occurred in late Pleistocene evolution of the genus. All three of the inferior molars are elliptical, and only the posterior enamel plate is present (as in all other genera of the tribe exceptPliogeomys).

The masseteric ridge of the mandible is well developed. In the late Pliocene speciesZ. persimilisandZ. minorthe mental foramen is directly beneath the anterior extension of the masseteric ridge, but in the living species,Z. trichopus, the foramen lies well anterior to the ridge. The basitemporal fossa in the livingspecies is well developed and deep; in the Pliocene species it is usually distinct but shallow (late Pliocene specimens ofZ. minor).

Referred species.—Three (two extinct and one living; the last has two subspecies):

*Zygogeomys minor(Gidley), 1922. U. S. Geol. Surv. Prof. Paper, 131:123, December 26. Type from Benson local fauna (late Pliocene), Cochise County, Arizona; also known from the Rexroad local fauna, Meade County, Kansas.

*Zygogeomys persimilisHay, 1927. Carnegie Inst. Washington Publ., 136. Originally described by Gidley, 1922 (U. S. Geol. Surv. Prof. Papers, 131:123, December 26) asGeomys parvidenswhich was preoccupied byG. parvidensBrown, 1908. Type from Curtis Ranch local fauna (middle Pleistocene), Cochise County, Arizona.

Zygogeomys trichopus trichopusMerriam, 1895. N. Amer. Fauna, 8:196, January 31. Type from Nahuatzen, Michoacán.

Zygogeomys trichopus tarascensisGoldman, 1938. Proc. Biol. Soc. Washington, 51:211, December 23. Type from 6 mi. SE Pátzcuaro, 8,000 ft., Michoacán.

GenusGeomysRafinesque

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