Genus—REITHRODON.[25]

Dentes primores ²⁄₂; inferioribus acutis, gracilibus, et anticè lævibus; superioribus gracilibus, anticè longitudinalitèr sulcatis.

Molares utrinque ³⁄₃ radicati; primo maximo, ultimo minimo: primo superiore plicas vitreas duas externè et internè alternatìm exhibente; secundo, et tertio, plicas duas externè, internè unam: primo inferiore plicas vitreas tres externè, duas internè; secundo, plicas duas externè, unam internè; tertio unam externè et internè, exhibentibus.

Artus inæquales: antipedes 4–dactyli, cum pollice exiguo: pedes postici 5–dactyli, digitis externis et internis brevissimis.

Ungues parvuli et debiles. Tarsi subtùs pilosi.

Cauda mediocris, pilis brevibus adpressis instructa.

Caput magnum, fronte convexo: oculis magnis: auribus mediocribus.

The present genus according to my views belongs to the familyMuridæ. The modifications of structure which have led me to separate it from the genusMusare as follows:

External characters.—The most conspicuous points of distinction between the external characters ofReithrodonandMus(if we regardM. rattus,M. decumanusorM. musculusas typical examples of that genus,) consist in the arched form of the head, the large size of the eyes, the stout form of the body, and the upper incisors being grooved. The ears, tail and feet are more denselyclothed with hairs, and the tarsus is covered with hair beneath,—at least the hinder portion.

Cranium.—The skulls of the species of the present genus differ from those of the species ofMusin being proportionately shorter and broader, and more arched; the facial portion of the skull is larger, compared with the cranial, the space between the orbits is narrower, and the orbits are larger; the palate is narrower and the incisive foramina are more elongated and larger. The pterygoids approximate anteriorly, so that the posteriornaresare greatly contracted. As in the genusMusthe anterior root of the zygomatic arch is directed upwards from the plane of the palate, and forwards in the form of a thin plate, protecting an opening behind, which leads into the nasal cavity, and also forming the outer boundary both of the ant-orbital foramen, and a second opening whose outlet is directed upwards. This thin plate, however, is narrower than is usually found in the genusMus. The most striking differences observable in the lower jaw consist in the smaller size of the coronoid process, and its being curved outwards; the condyloid process is narrower, and the angle of the jaw, or descending ramus, approaches more nearly to a quadrate form—the posterior edge of the jaw is more deeply emarginated.

Dentition.—The incisors are narrow and compressed as in the genusMus, but they are less deep from front to back; those of the upper jaw (Plate 33. fig 2.b.) have each a distinct longitudinal groove, which is situated nearer to the outer than to the inner edge of the tooth. Close to the inner edge of each of these teeth an indistinct second longitudinal groove may be seen by means of a lens. The lower incisors are nearly equal in width to the upper.

The crowns of the molar teeth in the youngReithrodonare higher than inMus, and they are rootless; in the adult animal, however, they possess distinct roots. The folds of enamel form sigmoid flexures, are closely approximated to each other, and those of the opposite sides of the tooth meet.

Reithrodon cuniculoïdes,Waterh., Proceedings of the Zoological Society of London for February 1837, p. 30.

R. suprà griseus, flavo-lavatus, pilis nigris intermixtis; abdomine guláque pallidè flavis; natibus albis; pedibus albis; auribus mediocribus, intùs pilis flavis, extùs pilis pallidè flavis, obsitis; pone aures, notâ magnâ albescenti-flavâ; caudâ corpore breviore, suprà pallidè fuscâ, subtùs albâ.

Description.—Head rather large and arched; ears moderate; tail nearly as longas the body; tarsi rather long; fur long and very soft. General tint of the upper parts of the body grayish brown, with a considerable admixture of yellow; of the sides of the body grayish tinted with yellow; the lower portion of the cheeks, and the lower half of the sides of the body are of a delicate yellow colour; the under parts of the head and body are yellowish white; the fore part of the thighs is whitish; the rump, feet, and tail are white, excepting the upper surface of the latter, which is brown; behind each ear there is a patch of yellowish white hairs. The ears are tolerably well clothed with hairs; those on the inner side are for the most part of a yellow colour, but towards the posterior margin they are brown; externally, the hairs are also yellow, excepting on the fore part, where they are dusky brown. The hairs of the moustaches are very long and numerous; black at the base, and grayish at the apex. The feet are well clothed with hairs which cover and nearly hide the claws; the under side of the tarsus is clothed with grayish brown hairs. The tail is well clothed with tolerably long hairs which completely hide the scales. The hairs on the back are of a deep gray colour at the base, broadly annulated with yellow near the apex, and dusky at the apex: the longer hairs are black: on the throat and belly the hairs are deep gray at the base, and broadly tipped with pale yellow—towards the cheeks and sides of the body with a deeper yellow. The incisors are yellow.

Habitat, Patagonia, (April and January).

In the arched form of the head this little animal bears considerable resemblance to a young rabbit, a resemblance which has struck almost all who have seen it, I have therefore applied to it the specific nameCuniculoïdes. The skull is figured in Plate 33, fig. 2.a., its dimensions are as follows:—

The molar teeth of the upper jaw are figured in Plate 33, fig. 2,c.and2,e; of the lower jaw, fig. 2,d. Fig. 2,b, represents the incisors of the upper jaw magnified. Fig. 21,a, Plate 34, represents the skull, viewed from beneath, fig. 21,b, is the side view of the same, and fig. 21,c, is the lower jaw.

“Specimens were procured at Port Desire, St. Julian, and Santa Cruz; at this latter place they were caught in numbers, (in traps baited with cheese,) both near the coast and on the interior plains. A specimen from Santa Cruz weighed 1336 grains. In the early part of January, there were young individuals at Port St. Julian.”—D.

Reithrodon typicus,Waterh., Proceedings of the Zoological Society of London for February 1837, p. 30.

R. vellere suprà pilis flavescenti-fuscis et nigrescentibus intermixtis composito; regione circa oculos, genis, lateribusque corporis auratis, pilis pallidè fuscis intermixtis; partibus inferioribus auratis; rhinario ad latera flavescenti-albo; auribus magnis, intùs pilis flavis, extùs flavis et fuscis, indutis; caudâ suprà pallidè fuscâ, subtùs sordidè albâ; pedibus albis.

Description.—Ears large; tarsi moderate; fur moderately long; general tint of the upper parts brown—of the upper surface of the head blackish; on the cheeks and flanks a rich yellow tint prevails; the under parts of the head and body are bright yellow; the feet are white; the tail is brownish above and dirty white beneath. The ears are tolerably well clothed with hairs, and these are of a yellowish colour, excepting on the fore part, externally, where they are brown; the tarsi are covered beneath with grayish brown hairs; the hairs of the moustaches are numerous and moderately long, black at the base and grayish at the apex. The hairs of the back are deep gray at the base, broadly annulated with yellow near the apex, and black at the apex; on the upper surface of the head the hairs are very narrowly annulated with yellow, hence a blackish hue prevails. The longer hairs on the back are black; the hairs of the throat and belly are gray at the base, and broadly tipped with yellow. The incisors are yellow.

Habitat, Maldonado, La Plata, (June).

This species is of a darker colour than the last, its ears are much larger and the tarsi are shorter. It has the same rabbit-like appearance. The molar teeth of the lower jaw are figured in Plate 33, fig. 4,a.

“This mouse, when alive, from its very large eyes and ears, had a singular appearance, somewhat resembling that of a little rabbit. It frequents small thickets in the open grassy savannahs near Maldonado, and was caught with facility by means of traps baited with cheese.”—D.

R. vellere longissimo et mollissimo; corpore suprà et ad latera cinereo, flavescenti-fusco lavato, subtùs flavescenti-albo; caudâ corpore breviore, suprà fuscâ, subtùs albâ: auribus parvulis: tarsis mediocribus.

Description.—Ears small; tail shorter than the body; tarsus moderate; fur long and extremely soft. General hue of the upper parts of the head and body ashy-brown; the lower part of the cheeks and sides of the body are of a delicate yellow colour; the under parts of the head and body and the rump are cream colour. The ears are blackish;[27]the tail is tolerably well clothed with longish hairs, which are, however, not so thickly set as to hide the scales—on the upper side they are blackish brown; on the sides and beneath they are white. The feet are white. All the fur is of a deep gray colour at the base; the hairs of the back are of a very pale yellow colour (almost white) near the tip, and brown at the tip; the longer hairs are black at the apex. The incisors are yellow; the hairs of the moustaches are numerous and very long—some of them are whitish, and others are black at the root, and gray at the apex.

Habitat, South shore of the Strait of Magellan, near the Eastern entrance.

This little animal was preserved in spirit, and has since been mounted, it isprobable, therefore, that the colours have been slightly changed. It is of a smaller size than either of the preceding species. Its fur is long, extremely soft, and somewhat resembles that of the Chinchilla. The ears are smaller, and the tail is shorter, and less densely clothed with hairs than inReithrodon cuniculoides. The skull (see Plate 43, fig. 20,a, 20,b, and 20,c,) differs in many respects from that of the species last mentioned. It is of a smaller size, the nasal portion is proportionately shorter and narrower, the incisive foramina are shorter; the pterygoid processes do not approximate so nearly at their base, and the pterygoid fossæ are very shallow, whereas inR. cuniculoidesthey are deep. In the skull of the animal just mentioned there are two distinct longitudinal grooves on the palate, which extend backwards from the incisive foramina, and terminate in two rather large and deep excavations: these excavations are in the palatine bone, and situated between the last molar teeth; they are separated from each other by a narrow, longitudinal, elevated ridge; a narrow ridge also separates them from the pterygoid fossæ. At the bottom of each of these hollows are several minute foramina, and in front of them there are two larger longitudinal foramina. InR. chinchilloides, the longitudinal grooves on the palate and the posterior hollows are shallow, and consequently much less distinct; the pterygoid fossæ are very nearly on the same plane as the palate, and are indicated only by a very slight depression. The incisor teeth are broader than inR. chinchilloides, and the molar teeth are proportionately smaller. The thin plate which forms the anterior root of the zygomatic arch is deeply emarginated in front inR. cuniculoides(see Plate 34, fig. 21,b.); but inR. chinchilloides, the anterior margin of this plate is nearly straight, (see Plate 34, fig. 20,c.)

In the form of the lower jaw of the two animals under consideration there are differences which will be more clearly understood upon comparing the figures. I will therefore merely notice one remarkable character which is found inR. cuniculoides, and that is, that the condyloid process is rather deeply concave on the inner side, a character which does not exist inR. chinchilloides, nor do I recollect having observed it in any other Rodent.

The principal dimensions of the skull ofR. chinchilloides, are as follows:—

General Observations upon the foregoing Species of Muridæ.

In the foregoing descriptions I have endeavoured to convey an idea of the characters of the species of mice submitted to me for examination and description, by Mr. Darwin: there are, however, some points upon which I have been silent in my descriptions. I allude to the characters observable in the dentition. I have omitted to notice the various modifications in the structure of the molar teeth, because I found it would lengthen the descriptions to no good purpose, inasmuch as of almost all the species I have made outlines of the molars, which will convey a more clear idea than any verbal description can do.

Upon an inspection of the Plates, it will be seen, that by far the greater portion of the teeth figured, may be referred to one particular type of form or pattern, and that this pattern does not agree with that observed in the molars ofMus Rattus,M. decumanus, orM. musculus, whilst these three species agree essentially with each other.

In the young Black Rat (Mus Rattus), before the teeth are worn, the two anterior molar teeth, on either side of the upper jaw, present three longitudinal rows of tubercles, a central series of larger tubercles, and on each side of these, a row of smaller ones. The front molar has three of the larger tubercles arranged along the middle of the tooth; three smaller ones on the outer side, and two, on the inner side. The second molars have two central tubercles, two outer, and two inner ones. The posterior molar is nearly round, the body of the tooth consists of three principal tubercles, and one small tubercle, situated on the inner and anterior portion of the tooth.

The corresponding teeth in the young ofMus bimaculatuspresent a very different appearance; the molars, instead of having three longitudinal rows of tubercles, have only two. An idea of the appearance of these teeth may be formed by removing the inner row of tubercles from the molars ofMus rattus. We should then have, as inMus bimaculatus, molars of a narrower form, the first tooth presenting six tubercles, the second, four; and the posterior tooth devoid of the small inner lobe; the opposing tubercles of each tooth, however, inM. bimaculatus, are of equal size.

The molars of the lower jaw ofMus bimaculatusagree with those ofM. Rattusas to the number of tubercles which they possess; they are, however, proportionately longer and narrower, and, when a little worn, these teeth, as well as those of the upper jaw, differ considerably from those ofM. Rattus. In the last named animal, when the molars are slightly worn, the ridges of enamel run completely across the tooth, as in Figs. 18 and 19, Plate 34. Such is not the caseinM. bimaculatusat any age. As soon as the molar teeth are worn, the folds of enamel penetrate the body of the tooth on each side, and those of one side alternate with those of the other,—in fact, they very nearly resemble those of theHamsters(Cricetus).

I have selected the molar teeth ofMus RattusandM. bimaculatusfor comparison, since I happened to possess specimens displaying both the young and adult states of each. But had I selected, on the one hand, almost any of the species brought from South America by Mr. Darwin, and, on the other hand, theMus musculusorM. decumanus, I should have had to point out the same distinctions—the former agreeing in dentition withM. bimaculatus, and the latter withM. Rattus.

The differences pointed out, between the molar teeth ofMus Rattusand those ofM. bimaculatus, I cannot but consider as important, since all the Old World species ofMuswhich I have yet had an opportunity of examining (and they are numerous) agree essentially with the former, whilst the onlyMusfrom S. America (exceptingM. MusculusandM. decumanus, which are carried in ships to all parts of the world) in which I have as yet found molar teeth like those ofM. Rattus, is theMus Maurus, and this it has been stated is possibly a variety ofM. decumanus.

Although as yet I have not met with species in the Old World possessing the characters of the South AmericanMuridæ, among those of North America, several have come under my observation. TheMus leucopus,Symidon hispidum, and the species ofNeotomacertainly belong to the same group,[28]as does also the species of the Galapagos Islands, described in this work under the nameGalapagoensis.

These considerations have induced me to separate the South American mice from those of the Old World,—or rather from that group of whichM. decumanusmay be regarded as the type,—and to place them, together with such North American species as agree with them in dentition, in a new genus bearing the nameHesperomys.[29]

Whether this group be confined to the Western hemisphere or not, I will not venture to say, but I think I may safely affirm that that portion of the globe is their chief metropolis.

The species of the genusHesperomys, which depart most from the type—whose dentition is least like figs. 5,a, and 5,b, Plate 33., or 6,a, and 6,b, of thesame Plate—recede still farther from the genusMus, and approach more nearly (as regards the dentition) to theArvicolidæ. Among the species here described I may mention as examples,M. griseo-flavus,M. zanthopygus, andM. Darwinii;—see the molar teeth figured in Plate 34. figs. 15, 16, and 17,—and among the North American species, those constituting the genusNeotoma. The latter make by far the nearest approach to theArvicolidæof any which have yet come under my observation, not only in the dentition, but in the form of the skull and the large size of the coronoid process of the lower jaw; there is, nevertheless, a tolerably well marked line of distinction between the crania of theArvicolidæandNeotoma.

The skulls of the animals belonging to the generaCastor,Ondatra,Arvicola,Spalax, andGeomys, which constitute the principal groups of the familyArvicolidæ, when compared with those of the familyMuridæ, present, among others, the following distinctive characters.

The temporalfossæare always much contracted posteriorly, by the great anterior and lateral development of the temporal bones; the plane of the intermolar portion of the palate is below the level of the anterior portion; the coronoid process of the lower jaw is very large, the articular portion of the condyloid process is proportionately broad; the descending ramus, or posterior coronoid process, is so situated that its upper portion terminates considerably above the level of the crowns of the molars; this same process is generally[30]directed outwards from the plane of the horizontal ramus. The incisor teeth of theArvicolidædiffer from those of theMuridæin being proportionately broader and less deep from front to back—they are not laterally compressed as inMus. The molar teeth are rootless,[31]and the folds of enamel are the same throughout the whole length of the tooth; whereas inMusthey enter less and less deeply into the body of the tooth as we recede from the crown, and towards the base of the visible portion (the tooth being in its socket) the indentations of the enamel are obliterated.

Now in the species ofHesperomys, the molar teeth are always rooted, and in the form of the skull and the lower jaw they agree with theMuridæ, and do notpresent the characters above pointed out as distinguishing theArvicolidæ, and as regards the cranium and lower jaw, it is only in the genusNeotomathat any approach is evinced.

Of the various groups of the orderRodentiafound in South America, theSciuridæ, so far as I am aware, are chiefly confined to the more northern parts, and do not occur in the most southern; theMyoxidæ,Gerboidæ, andArvicolidæare wanting. The species of the familyMuridæbelong to different sections to those of the Old World. Of theLeporidæI am acquainted only with one well established species—theLepus Braziliensis, which however is not found “in tota America Australi,” as Fischer says, there being no Hare yet found in the more southern parts, where theCaviesandChinchillasappear to take their place. The remaining South American Rodents—certain species ofHystricidæ, the genera,Echimys,Dasyprocta,CælogenysandMyopotamus, together with theOctodontidæandChinchillidæ, all possess a peculiar form of skull and of the lower jaw, (more or less approaching to figs. 1, Plate 33, and figs. 23, Plate 34.) which I have described in the “Magazine of Natural History,” for February 1839, and which is rarely found in the North American, or Old World Rodents. In enumerating the above groups, I omitted theCaviidæ, because in the form of the lower jaw they differ somewhat from the rest—they possess, in fact, a form of lower jaw peculiar to themselves; but in the Chinchillas[32]the transitions between one form and the other are found.

The South AmericanMuridæ, which form the chief part of Mr. Darwin’s collection, were none of them procured further north than latitude 30°, with the exception of those from the Galapagos Archipelago. The species occur at the following localities.

WEST COAST OF SOUTH AMERICA.Galapagos Archipelago.Mus Jacobiæ.—— Galapagoensis.Coquimbo.Mus longipilis.—— Renggeri.—— Darwinii.Valparaiso.Mus Renggeri.——decumanus.Concepcion.Mus longicaudatus.Chiloe and Chonos Archipelago.Mus brachiotis.EAST COAST OF SOUTH AMERICA.Maldonado.Musdecumanus.—— maurus.——Musculus.—— tumidus.—— nasutus.—— obscurus.—— arenicola.—— bimaculatus.—— flavescens.Reithrodon typicus.Buenos Ayres.—— Musdecumanus.Bahia Blanca.Mus Braziliensis.—— elegans.—— gracilipes.Rio Negro.Mus griseo-flavus.Port Desire.Mus canescens.St. Julian.Reithrodon cuniculoïdes.—— xanthopygus.Reithrodon cuniculoïdes.Santa Cruz.Mus canescens.—— micropus.—— xanthopygus.Reithrodon cuniculoïdes.Falkland Islands.Musdecumanus.——Musculus.Straits of Magellan.Mus xanthorhinus.—— Magellanicus.Reithrodon chinchilloïdes.

WEST COAST OF SOUTH AMERICA.

Galapagos Archipelago.

Mus Jacobiæ.—— Galapagoensis.

Mus Jacobiæ.

—— Galapagoensis.

Coquimbo.

Mus longipilis.—— Renggeri.—— Darwinii.

Mus longipilis.

—— Renggeri.

—— Darwinii.

Valparaiso.

Mus Renggeri.——decumanus.

Mus Renggeri.

——decumanus.

Concepcion.

Mus longicaudatus.

Chiloe and Chonos Archipelago.

Mus brachiotis.

EAST COAST OF SOUTH AMERICA.

Maldonado.

Musdecumanus.—— maurus.——Musculus.—— tumidus.—— nasutus.—— obscurus.—— arenicola.—— bimaculatus.—— flavescens.Reithrodon typicus.

Musdecumanus.

—— maurus.

——Musculus.

—— tumidus.

—— nasutus.

—— obscurus.

—— arenicola.

—— bimaculatus.

—— flavescens.

Reithrodon typicus.

Buenos Ayres.

—— Musdecumanus.

Bahia Blanca.

Mus Braziliensis.—— elegans.—— gracilipes.

Mus Braziliensis.

—— elegans.

—— gracilipes.

Rio Negro.

Mus griseo-flavus.

Port Desire.

Mus canescens.

St. Julian.

Reithrodon cuniculoïdes.—— xanthopygus.Reithrodon cuniculoïdes.

Reithrodon cuniculoïdes.

—— xanthopygus.

Reithrodon cuniculoïdes.

Santa Cruz.

Mus canescens.—— micropus.—— xanthopygus.Reithrodon cuniculoïdes.

Mus canescens.

—— micropus.

—— xanthopygus.

Reithrodon cuniculoïdes.

Falkland Islands.

Musdecumanus.——Musculus.

Musdecumanus.

——Musculus.

Straits of Magellan.

Mus xanthorhinus.—— Magellanicus.Reithrodon chinchilloïdes.

Mus xanthorhinus.

—— Magellanicus.

Reithrodon chinchilloïdes.

Back to Index Next