Fig. 90. Half front view[165]of the skulls of a Tasmanian Wolf(Thylacinus cynocephalus) (to the left) ×3/8;and of ahairy-nosed Wombat(Phascolomys latifrons) (to the right) ×3/8. (Camb. Mus.)1. premaxillae.7. coronoid process of the2. nasal.mandible.3. frontal.8. lachrymal foramen.4. infra-orbital foramen.i. 1. first upper incisor.5. lachrymal.C. canine.6. jugal.
Fig. 90. Half front view[165]of the skulls of a Tasmanian Wolf(Thylacinus cynocephalus) (to the left) ×3/8;and of ahairy-nosed Wombat(Phascolomys latifrons) (to the right) ×3/8. (Camb. Mus.)1. premaxillae.7. coronoid process of the2. nasal.mandible.3. frontal.8. lachrymal foramen.4. infra-orbital foramen.i. 1. first upper incisor.5. lachrymal.C. canine.6. jugal.
Fig. 90. Half front view[165]of the skulls of a Tasmanian Wolf
(Thylacinus cynocephalus) (to the left) ×3/8;and of a
hairy-nosed Wombat
(Phascolomys latifrons) (to the right) ×3/8. (Camb. Mus.)
Marsupialia.The skulls of the various types of the Marsupials frequently bear a strong superficial resemblance to those of some of the different groups of placental mammals. Thus the skull of the Dasyuridae resembles that of the Carnivora, the resemblance being most marked between the skulls ofThylacinusand the dog. The skull ofNotoryctesis strongly suggestive of that of an Insectivore, and that of other Marsupials such as the wombat, recalls equally the characteristic features of a Rodent's skull. But, however much they may differ from one another, the skulls of all Marsupials agree in the following respects. (1) The brain cavity, and especially the cerebral fossa, has a very small comparative size. (2) The nasals are always large, and the mesethmoid is extensively ossified, and terminated by a prominent vertical edge. (3) Processes from the jugal and frontal in living forms never meet and enclose the orbit, but the zygomatic arch is always complete. (4) The jugal always extends back to form part of the glenoid fossa. (5) The lachrymal canal opens either external to or upon the margin of the orbit, and the nasal processes of the premaxillae never quite reach the frontals. (6) The posterior part of the palate is commonly pierced by large oval vacuities. (7) The tympanic is small and never fused to the bones of the cranium. (8) The carotid canal perforates the basisphenoid and not the tympanic bulla. (9) The optic foramen and sphenoidal fissure are confluent. (10) In every case exceptTarsipesthe angle of the mandible is more or less inflected.
The skull of the extinctThylacoleodiffers from that of all other Marsupials in the fact that the postorbital bar is complete. The hyoid is constructed on much the same plan in all Marsupials. It consists of a small basi-hyal, a pair of broad cerato-hyals, and a pair of strong thyro-hyals. The epi-hyals and stylo-hyals are generally unossified.
Edentata.In Sloths (Bradypodidae) the sutures become early obliterated, the cranial portion of the skull is rather high, and the facial portion very short. The lachrymal is very small, and its canal opens outside the orbit. The zygomatic arch is incomplete, and the jugal (fig. 91, 5) is curiously forked, but in a manner differing in the two genera. The premaxillae are very small,—inBradypusquite vestigial. The mandible is well developed, the angle being specially marked inBradypus.InCholoepusthe symphysial part is drawn out in a somewhat spout-like manner (fig. 91, 6). In both genera the thyro-hyals are ankylosed with the basi-hyal.
Fig. 91. Skull of a two-fingered Sloth(Choloepus didactylus) × ½. (Camb. Mus.)1. anterior nares.4. angle of the mandible.2. postorbital process of the5. jugal.frontal.6. spout-like prolongation of the3. coronoid process.mandible.
Fig. 91. Skull of a two-fingered Sloth(Choloepus didactylus) × ½. (Camb. Mus.)1. anterior nares.4. angle of the mandible.2. postorbital process of the5. jugal.frontal.6. spout-like prolongation of the3. coronoid process.mandible.
Fig. 91. Skull of a two-fingered Sloth
(Choloepus didactylus) × ½. (Camb. Mus.)
InMegatheriumthe general appearance of the skull is distinctly sloth-like, but the facial portion is more elongated, partly owing to the development of a prenasal bone, and the zygomatic arch is complete. The mandible is very deep in the middle, and is drawn out into a long spout-like process in front.
Anteaters (Myrmecophagidae) have a much modified skull, and this is especially the case in the Great Anteater,Myrmecophaga. The skull is smooth and evenly-rounded, in these respects recalling that ofEchidna, but it is longer and tapers much more gradually than inEchidna. The occipital condyles are remarkably large. The premaxillae are small, and the long rostrum is chiefly composed of the maxillae and nasals with the mesethmoid and vomer. The zygomatic arch is incomplete, and there is no trace of a separation between the orbit and the temporal fossa. The palate is much elongated,the pterygoids meeting in the middle line just like the palatines. The mandible is very long and slender, there being no definite coronoid process, and a short and slight symphysis. The hyoid arch is noticeable for the length of the anterior cornu.
In the Armadillos (Dasypodidae) the skull varies a good deal in shape, but the facial portion is always tapering and depressed. The zygomatic arch is complete. InDasypusandChlamydophorusthe tympanic bulla is well ossified.
In the Glyptodontidae the skull is very short and deep; the zygomatic arch is complete, and has a long downwardly projecting maxillary process. The mandible is massive, and has a very high ascending portion.
In the Manidae the skull is smooth and rounded, the zygomatic arch is incomplete, and the orbit is inconspicuous. The palate is long and narrow, but the pterygoids do not take part in its formation. The mandible is slightly developed and has no angle or coronoid process.
InOrycteropusthe zygomatic arch is complete, and there is a small postorbital process to the frontal. The mandible is well-developed, having a coronoid process and definite ascending portion, and the hyoid is well ossified.
Sirenia.The skull, and especially the brain case of all Sirenia, is remarkable for the general density of the component bones, which, though often very thick, are without air sinuses. It is noticeable also for the roughness of the bones, and the irregular manner in which they are united together.
Fig. 92. Lateral view of the skull ofRhytina stelleri×1/8.(Brit. Mus.)1. frontal.7. pterygoid process of the2. parietal.alisphenoid.3. zygomatic process of the8. jugal.squamosal.9. premaxillae.4. squamosal.10. angle of the mandible.5. exoccipital.11. maxillae.6. occipital condyle.
Fig. 92. Lateral view of the skull ofRhytina stelleri×1/8.(Brit. Mus.)1. frontal.7. pterygoid process of the2. parietal.alisphenoid.3. zygomatic process of the8. jugal.squamosal.9. premaxillae.4. squamosal.10. angle of the mandible.5. exoccipital.11. maxillae.6. occipital condyle.
Fig. 92. Lateral view of the skull ofRhytina stelleri×1/8.
(Brit. Mus.)
The cranial cavity is decidedly small, the reduction being specially noticeable in the cerebral fossa, which is not much larger than the cerebellar fossa. The foramen magnum is large, and the dorsal surface of the cranium narrow. The zygomatic arch is very strongly developed, the squamosal (fig. 92, 4) being especially prominent, and being drawn out not only into the zygomatic process, but also into a large post-tympanic process which articulates with the exoccipital. At the sideof the skull between the squamosal, supra-occipital and exoccipital, there is a wide vacuity in the cranial wall, partially filled up by the very large periotic, which is ankylosed to the tympanic, but is not united to any other bones of the skull. The foramen lacerum medium is confluent with the foramen lacerum anterius, and the two together form an enormous vacuity on the floor of the skull, bounded chiefly by the exoccipital, basi-occipital, alisphenoid and squamosal. The jugal (fig. 92, 8) is large and inManatussends up a strong process, which nearly or quite meets the postorbital process of the frontal, completing the orbit. In the other Sirenia the orbit is completely confluent with the very large temporal fossa. The lachrymal inManatusis very small, but is larger inHalicore. The premaxillae (fig. 92, 9) are large, but smaller inManatusthan in the othergenera, in all of which they are curiously bent down in front. Their upper margin forms the anterior border of a very large aperture lying high on the roof of the skull and extending back for a considerable distance. This aperture is formed by the union of the two anterior nares. The nasals are quite vestigial or absent, and the narial aperture is bounded above by the frontals; in its floor are seen the slender vomer and large mesethmoid. The palate is long and narrow, and formed mainly by the maxillae; behind it there is a large irregular process formed by the union of the palatine, pterygoid, and pterygoid plate of the alisphenoid. The mandible is very massive and has a very high ascending portion, a rounded angle (fig. 92, 10), and a prominent coronoid process; the two rami are firmly ankylosed together. The hyoid consists principally of the broad flat basi-hyal; the anterior cornua are but slightly ossified, while the thyro-hyals are not ossified at all.
Cetacea.The skull in all Cetacea, especially in the Odontoceti, is a good deal modified from the ordinary mammalian type.
In theArchaeocetithis modification is less marked than in either of the other suborders. The nasals and premaxillae are a good deal larger than they are in living forms, and the anterior nares are placed further forward. The maxillae do not extend back over the frontals, and there is a well-marked sagittal crest.
In theMystacocetithe skull is always quite bilaterally symmetrical, and is not so much modified from the ordinary mammalian type as in the Odontoceti. The parietals are not, as in the Odontoceti, separated by a wide interparietal, but meet; they are, however, hidden under the very large supra-occipital. The nasals are developed to a certain extent, and the nares, though placed very far back and near the top of the head, terminate forwardly-directed narial passages. Turbinal bones are also developed to some extent; this fact, and the occurrence of a definite though small olfactory fossa constitutingimportant distinctions from the Odontoceti. The maxillae are large, but do not extend back to cover the frontals as in the Odontoceti. The zygomatic process of the squamosal is very large. The mandibular rami are not compressed, but are rounded and arched outwards, and never meet in a long symphysis.
Odontoceti.The skull departs widely from the ordinary mammalian type. The following description will apply to any of the following genera of the Delphinidae,Phocaena,Globicephalus,Lagenorhynchus,Delphinus,Tursiops,Prodelphinus,Sotalia.
The upper surface of the skull is more or less asymmetrical. The cerebral cavity is high, short and broad; and formed mainly by the cerebral fossa, the olfactory fossa being entirely absent. The supra-occipital (fig. 93, 3) is very large, and forms much of the posterior part of the roof of the skull. It has the interparietal (fig. 93, 7) fused with it, and completely separates the two parietals. The frontal (fig. 93, 10) is large and laterally expanded, forming the roof of the orbit, but is almost completely covered by an extension of the maxillae. The zygomatic arch is very slender, and is mainly formed by a rod-like process from the jugal (fig. 93, 15), the zygomatic process of the squamosal being short and stout.
The nasal passages are peculiarly modified, instead of passing horizontally forwards above the roof of the mouth, they pass upwards and even somewhat backwards towards the top of the skull (fig. 93, 23). They are bounded laterally by two processes from the premaxillae, the left of which is shorter than the right. The nasal cavities are narrow and without turbinals and the nasals (fig. 93, 19) are almost as much reduced as in Sirenia.
Fig. 93. A, Lateral view, and B, Longitudinal section of the skullof a young Ca'ing Whale(Globicephalus melas) ×1/6. (Brit. Mus.)1. basi-occipital.13. periotic.2. exoccipital.14. squamosal.3. supra-occipital.15. jugal.4. basisphenoid.16. vomer.5. alisphenoid.17. palatine.6. parietal.18. pterygoid.7. interparietal.19. nasal.8. presphenoid.20. maxillae.9. orbitosphenoid.21. premaxillae.10. frontal.22. mandible.11. mesethmoid.23. anterior nares.12. tympanic.
Fig. 93. A, Lateral view, and B, Longitudinal section of the skullof a young Ca'ing Whale(Globicephalus melas) ×1/6. (Brit. Mus.)1. basi-occipital.13. periotic.2. exoccipital.14. squamosal.3. supra-occipital.15. jugal.4. basisphenoid.16. vomer.5. alisphenoid.17. palatine.6. parietal.18. pterygoid.7. interparietal.19. nasal.8. presphenoid.20. maxillae.9. orbitosphenoid.21. premaxillae.10. frontal.22. mandible.11. mesethmoid.23. anterior nares.12. tympanic.
Fig. 93. A, Lateral view, and B, Longitudinal section of the skull
of a young Ca'ing Whale(Globicephalus melas) ×1/6. (Brit. Mus.)
In front of the nasal openings the face is prolonged as anarrow beak or rostrum of varying length, formed by the maxillae and premaxillae surrounding the vomer and large mesethmoid (fig. 93, 11), which sends forwards a long partially cartilaginous process, and is fused behind with the presphenoid (fig. 93, 8). The basi-occipital (fig. 93, 1) too is fused with the basisphenoid. The foramen rotundum is confluent with the sphenoidal fissure, and the foramen ovale with the foramen lacerum medium and the foramen lacerum posterius. The palate is mainly formed by the maxillae; the premaxillae and palatines (fig. 93, 17), though both meet in symphyses, forming very little of it. The pterygoids vary in size in the different genera, sometimes as inLagenorhynchusandDelphinusmeeting in the middle line, sometimes as inPhocaenaandGlobicephalus(fig. 93, 18) being widely separated. The tympanic and periotic are not fused together, and the periotic has generally no bony union with the rest of the skull. The mandible is rather slightly developed, with the rami straight, compressed and tapering to the anterior end. The condyle is not raised at all above the edge of the ramus; the angle is rounded and the coronoid process is very small.Platanistahas a curiously modified skull; the rostrum and mandible are exceedingly long and narrow, and arising from the maxillae are two great plates of bone which nearly meet above.
In the Physeteridae the skull is raised into a very prominent crest at the vertex behind the nares. In front of this inHyperoödona pair of ridges occur, formed by outgrowths from the maxillae. In the old male these ridges reach an enormous size and almost meet in the middle line. InPhyseter, the Sperm whale, these ridges are not developed; the maxillae and premaxillae unite with the other bones of the crest enclosing an enormous half basin-shaped cavity, at the base of which are the very asymmetrical anterior narial apertures.
In all living Cetacea the hyoid has the same general shape, consisting firstly of a crescentic bone formed by the fusion of the thyro-hyals with the basi-hyal, and secondly of the anterior cornu formed principally by the strong stylo-hyal.
Ungulata.None of the distinctive characters separatingthe Ungulata from the other groups of mammals are drawn from the skull. But in the Ungulata vera as opposed to the Subungulata a distinguishing feature is found in the fact that the lachrymal and jugal form a considerable part of the side of the face, and that the jugal always forms the anterior part of the zygomatic arch, the maxillae taking no part in it.
Ungulata vera.
Artiodactyla.The skull in Artiodactyla differs from that in Perissodactyla in the fact that the posterior end of the nasal is not expanded and there is no alisphenoid canal.
The skulls in the different groups of Artiodactyla differ considerably from one another.
Fig. 94. A, Cranium and B, mandible of a Pig(Sus scrofa) ×1/5.(Camb. Mus.)1. jugal.11. anterior palatine foramen.2. postorbital process of the12. palatal plate of maxillae.frontal.13. coronoid process.3. zygomatic process of the14. mandibular condyle.squamosal.i1,i2,i3. first, second, and third4. supra-occipital.incisors.5. glenoid cavity.c.canine.6. occipital condyle.pm1,pm2,pm3,pm4. first,7. foramen magnum.second, third, and fourth8. paroccipital process of thepremolars.exoccipital.m1,m2,m3. first, second, and9. tympanic bulla.third molars.10. pterygoid.
Fig. 94. A, Cranium and B, mandible of a Pig(Sus scrofa) ×1/5.(Camb. Mus.)1. jugal.11. anterior palatine foramen.2. postorbital process of the12. palatal plate of maxillae.frontal.13. coronoid process.3. zygomatic process of the14. mandibular condyle.squamosal.i1,i2,i3. first, second, and third4. supra-occipital.incisors.5. glenoid cavity.c.canine.6. occipital condyle.pm1,pm2,pm3,pm4. first,7. foramen magnum.second, third, and fourth8. paroccipital process of thepremolars.exoccipital.m1,m2,m3. first, second, and9. tympanic bulla.third molars.10. pterygoid.
Fig. 94. A, Cranium and B, mandible of a Pig(Sus scrofa) ×1/5.
(Camb. Mus.)
The skull of the Pig[166]will be described as illustrative of the skull in the Suina. In the Pig as in most Artiodactyla the face is bent sharply down on the basicranial axis, the commencement of the vomer being situated below the mesethmoid instead of in front of it as in most skulls. The occipital region of the skull is small, and the line of junction of the supra-occipital and parietals is raised into a prominent occipital crest. The parietal completely fuses at an early stage with its fellow, and the exoccipital is drawn out into a long paroccipital process (fig. 94, A, 8). The frontal is large and broad and drawn out into a small postorbital process. The lachrymal too is large and takes a considerable part in forming the side of the face in front of the orbit, as does also the jugal, though to a less extent. The face is long and tapers much anteriorly. The nasals are long and narrow, as are the nasal processes of the premaxillae, which do not however reach the frontals. A prenasal ossicle is developed in front of the mesethmoid. The palate is long and narrow, the pterygoid (fig. 94, A, 10) is small, but the pterygoid process of the alisphenoid is prominent. The squamosal is small and has the tympanic fused with it; the tympanic is dilated below, forming a bulla (fig. 94, A, 9) filledwith cancellous bone, and above forms the floor of a long upwardly-directed auditory meatus. The mandible has a high ascending portion and a small coronoid process (fig. 94, B, 13). The hyoid differs from that of most Ungulates, the stylo-hyal being very imperfectly ossified.
Fig. 95. Mandible of a Hippopotamus(H. amphibius) ×1/7.(Camb. Mus.)The second incisor of the left side is missing and the crowns of the grinding teeth are much worn.1. condyle.c.canine.2. coronoid process.pm3. third premolar.3. mental foramina.m1,m3. first and third molar.i1,i2. first and second incisors.
Fig. 95. Mandible of a Hippopotamus(H. amphibius) ×1/7.(Camb. Mus.)The second incisor of the left side is missing and the crowns of the grinding teeth are much worn.1. condyle.c.canine.2. coronoid process.pm3. third premolar.3. mental foramina.m1,m3. first and third molar.i1,i2. first and second incisors.
Fig. 95. Mandible of a Hippopotamus(H. amphibius) ×1/7.
(Camb. Mus.)
The second incisor of the left side is missing and the crowns of the grinding teeth are much worn.
InHippopotamusthe skull though essentially like that of the pig is much modified in detail. The brain cavity is very small, while the jaws are immensely developed. The face contracts in front of the orbits and then expands again greatly, to lodge the enormous incisor and canine teeth. The postorbital bar is complete or nearly so, and the orbits project curiously outwards and slightly upwards; the lachrymal is thin andmuch dilated. The squamosal is drawn out into a postglenoid process, and the hamular process of the pterygoid is prominent. The tympanic bulla is filled with cancellous bone. The mandible is enormously large, the symphysis is long, the angle much expanded and drawn out into a process which projects outwards and forwards.
Among extinct forms related to the Suina,Cyclopidiusis noticeable for having large vacuities in the lachrymo-nasal region, whileCotylopshas the postorbital bar complete; both these forms are from the North American Miocene.
In the Tylopoda and Tragulina the skull resembles in most respects that of the Ruminants, shortly to be described; but it is allied to that of the Suina in having the tympanic bulla filled with cancellous bone. The tympanic bulla is better developed in the Tragulina than in most Ungulates.
Among Ruminants, the Bovidae, that large group including the Oxen, Sheep, and Antelopes, as a rule have the face bent on the basicranial axis much as in the Suina. The parietals are generally small and early coalesce, the frontals are large and are usually drawn out into horn cores, which are however absent in the skulls of some domestic varieties of sheep and oxen, and also in some of the earlier extinct forms of Bovidae. These horn cores are formed internally of cancellous bone, and on them the true epidermal horns are borne. In young animals there is a distinct interparietal, but this early fuses with the supra-occipital, and in the oxen also with the parietals. The occipital crest is generally well marked, but in the genusBosbecomes merged in a very prominent straight ridge running between the two horn cores; this ridge, which contains air cells communicating with those in the horn cores, is not nearly so well marked inBison. There is often, as inGazella, a vacuity on the side of the face between the nasal, frontal, lachrymal, and maxillae, but this is not found in oxen or sheep. The premaxillae are small, the nasals are long and pointed, and the turbinals are much developed. The Saiga antelopehas a curiously specialised skull; the nasals are absent or have coalesced with the frontals and the anterior nares are enormously large. In all Ruminants the lachrymal is large and forms a considerable part of the side of the face; it often bears a considerable depression, thesuborbitalorlachrymal fossa, well seen in most of the smaller antelopes. The postorbital bar is complete, and the orbit is prominent and nearly circular. The palatines and pterygoids are moderately large, and the pterygoids have a backwardly-projecting hamular process. The squamosal is small, but has a postglenoid process. The tympanic is not fused to the periotic and has a small bulla not filled with cancellous bone. There is a large paroccipital process to the exoccipital and the mandible has a long slender coronoid process.
In the Cervidae and Giraffidae the face is not bent down on the basicranial axis as it is in the Bovidae. The frontals are drawn out, not into permanent horn cores as in the Bovidae, but into short outgrowths, the pedicels, upon which in the Cervidae long antlers are annually developed. Theseantlersare outgrowths of bone, and are covered during development by vascular integument, which dries up and peels off when growth is complete. Every year they are detached, by a process of absorption at the base, and shed. They may occur in both sexes, as in the Reindeer, but as a rule they are found only in the male. They are generally more or less branched, and are sometimes of enormous size and weight, as in the extinctCervus megaceros. In young animals they are always simple, but become annually more and more complicated as the animal grows older.
In the Giraffe the frontals bear a small pair of bony cores, which are at first distinct, but subsequently become fused to the skull. In the alliedSivatherium, a very large form from the Indian Pliocene, the skull bears two pairs of bony outgrowths, a pair of short conical outgrowths above the orbits, and a pair of large expanded outgrowths on the occiput.
The opening of the lachrymal canal is commonly doubleand the lachrymal fossa is large in the Cervidae and the Giraffidae exceptSivatherium. The vacuity between the frontal, lachrymal, maxillae, and nasal is specially large.
The hyoid of Ruminants is noticeable for the development of the anterior cornua, which include stout and short cerato-hyals and epi-hyals, long and strong stylo-hyals and large tympano-hyals which are more or less imbedded in the tympanics.
Perissodactyla.In the skull of Perissodactyles an alisphenoid canal is found and the nasals are expanded behind. Among the living animals belonging to this group the skull least modified from the ordinary type is that inRhinoceros. In this form the skull is considerably elongated, the facial portion being very large. The occipital region is elevated, but the cranial cavity is small, the boundary line between the occipital and parietal regions being drawn out into a prominent crest, which is occupied by air cells. There is no postorbital process to the frontal, and the orbit is completely confluent with the temporal fossa. The nasals are fused together and are very strongly developed, extending far forwards, sometimes considerably beyond the premaxillae. In some extinct species, such asElasmotheriumand the Tichorhine Rhinoceros,R. antiquitatis, the mesethmoid is ossified as far forwards as the end of the nasals. The nasals are arched and bear one or two roughened surfaces to which the great nasal horns are attached. The premaxillae are very small and the pterygoids are slender. The palate is long, narrow, and deeply excavated behind. The postglenoid process of the squamosal is well developed, and generally longer than the paroccipital process of the exoccipital. The tympanic and periotic are both small and are fused together. The condyle of the mandible is very wide, the angle rounded, and the coronoid process moderately developed.
In the Titanotheriidae, a family of extinct Perissodactyla from the Miocene of North America, the occipital region is much elevated, as is also the fronto-nasal region, the nasals (perhapsonly in the male) bearing a pair of blunt bony outgrowths. Between these two elevated regions the skull is much depressed. The cranial cavity is very small, the orbit confluent with the temporal fossa, and the zygomatic arch massive.
InTapirusthe orbit and temporal fossa are confluent. The nasals are small, wide behind and pointed in front, and are supported by the mesethmoid; the anterior nares are exceedingly large and their lateral boundaries are entirely formed by the maxillae. The postglenoid and post-tympanic processes of the squamosal are large. The periotic is not fused to the squamosal or to the small tympanic. The mandible is large and has the angle much developed and somewhat inflected.
Palaeotherium, which lived in early Tertiary times, has a skull much like that of the Tapir, especially as regards the nasal bones.
In the Horse and its allies (Equidae) the facial portion of the skull is very large as compared with the cranial portion, the nasals and nasal cavities being specially large. In the living species of the genusEquusthere is no fossa between the maxillae and lachrymal, but it occurs in some extinct species. The lachrymal and jugal form a considerable part of the side of the face; and the orbit though small is complete and prominent. The postorbital bar is formed by a strong outgrowth from the frontal, which unites with a forward extension of the squamosal. The squamosal may extend forwards and form part of the wall of the orbit, a very unusual feature, as in most mammals the squamosal stops before the postorbital bar. The palate is narrow and excavated behind as inRhinoceros; the palatines take very little part in its formation. The glenoid surface for the articulation of the mandible is very wide. The squamosal gives rise to small postglenoid and post-tympanic processes, and the exoccipital to a large paroccipital process. The tympanic and periotic are ankylosed together, but not to any other bones.
In theSubungulata, the lachrymal and jugal do not formany considerable part of the side of the face, and the maxillae commonly takes part in the formation of the zygomatic arch.
Toxodontia.The skull in the Toxodontia shows several Artiodactyloid features, while the manus and pes are of a more Perissodactyloid type. The Artiodactyloid features are (1) the absence of an alisphenoid canal, (2) the fact that the palate is not excavated behind, and that the palatines form a considerable part of it, and (3) the fusion of the tympanic to the squamosal and exoccipital, forming the floor of an upwardly directed auditory meatus. The frontal has a fairly well developed postorbital process, but the orbit is confluent with the temporal fossa. The premaxillae is well developed, as is the paroccipital process of the exoccipital, especially inTypotherium. The mandible has a rounded angle and a coronoid process of moderate size. InTypotheriumthe ascending portion is very massive.
Condylarthra.As far as is known the skull of these generalised Ungulates is depressed, and is frequently marked by a strong sagittal crest. The cranial cavity is small, the cerebral fossa inPhenacodusbeing exceptionally small. The orbit is completely confluent with the temporal fossa.
Hyracoidea.The skull ofProcaviaresembles that of Perissodactyles more than that of any other Ungulates, but differs strongly in the comparatively small size of its facial portion. The posterior portion of the cranium is rather high, the occipital plane being nearly vertical. There is a small interparietal. The nasals are wide behind, and the zygomatic arch is strongly developed, its most anterior part being formed by the maxillae. The jugal and parietal give rise to postorbital processes which sometimes meet, but as a rule the orbit is confluent with the temporal fossa; it is very uncommon for the parietal to give rise to a postorbital process, and even inProcaviathe frontal often forms part of the process. The alisphenoid canal, and postglenoid and paroccipital processes are well developed. The tympanic bulla is large and theperiotic and tympanic are fused together, but not as a rule to the squamosal. The ascending portion of the mandible is very high and broad, the angle rounded and the coronoid process moderate in size. The hyoid is singular, there is a large flat basi-hyal prolonged laterally into two broad flattened thyro-hyals. Articulating with its anterior end are two large triangular cerato-hyals, which are drawn out into two processes meeting in the middle line.
Amblypoda.In the Uintatheriidae (Dinocerata) the skull has a very remarkable character, being long and narrow and drawn out into three pairs of rounded protuberances, a small pair on the nasals, a larger pair on the maxillae in front of the orbits, and the largest pair on the parietals. The cranial cavity, and especially the cerebral fossa, is extraordinarily small. The orbit is not divided behind from the temporal fossa. The mandible has a prominent angle, and a long curved coronoid process; its symphysial portion bears a curious flattened outgrowth to protect the great upper canines.
InCoryphodonthe skull is of a more normal character, being without the conspicuous protuberances. The cranial cavity though very small is not so small as inUintatherium.
Fig. 96. Skull of a young Indian Elephant(Elephas indicus),SEENfrom the right side, the roots of the teeth have been exposed.×1/8. (Camb. Mus.).1. exoccipital.14. postorbital process of the2. parietal.frontal.3. frontal.15. lachrymal.4. squamosal.16. pterygoid process of the5. jugal.alisphenoid.6. premaxillae.i1. incisor.7. maxillae.mm3.,mm4. third and fourth9. supra-occipital.milk molars.13. basi-occipital.m1. first molar.
Fig. 96. Skull of a young Indian Elephant(Elephas indicus),SEENfrom the right side, the roots of the teeth have been exposed.×1/8. (Camb. Mus.).1. exoccipital.14. postorbital process of the2. parietal.frontal.3. frontal.15. lachrymal.4. squamosal.16. pterygoid process of the5. jugal.alisphenoid.6. premaxillae.i1. incisor.7. maxillae.mm3.,mm4. third and fourth9. supra-occipital.milk molars.13. basi-occipital.m1. first molar.
Fig. 96. Skull of a young Indian Elephant(Elephas indicus),SEEN
from the right side, the roots of the teeth have been exposed.×1/8. (Camb. Mus.).
Proboscidea.The character of the skull in the young elephant differs much from that in the old animal. In very young individuals the skull is of a normal character, and the cranial cavity is distinctly large in proportion to the bulk of the skull. But as the animal gets older, while its brain does not grow much, the size of its trunk and especially of its tusks increases greatly; and consequently the skull wall is required to be of very great superficial extent in order to afford space for the attachment of the muscles necessary for the support of these heavy weights. This increase in superficial extent is brought about without much increase in weight of bone by the development of an enormous number of air cells in nearly all the bones of the skull; sometimes, as in the case of the frontal, separating the inner wall of the bone from the outer, by asmuch as a foot. This development of air cells is accompanied by the obliteration of the sutures between the various bones. The most noticeable point with regard to the cranial cavity is the comparatively large size of the olfactory fossa. The supra-occipital (figs. 96 and 97, 9) is large—exceedingly large in the adult skull; the parietals (figs. 96 and 97, 2) are also very large. The frontals send out small postorbital processes, but these do not meet processes from the small jugal, whichforms only the middle part of the slender zygomatic arch, the anterior part being formed by the maxillae. The lachrymal (fig. 96, 15) is small and lies almost entirely inside the orbit. The anterior narial aperture (fig. 97, 8) is wide and directed upwards, opening high on the anterior surface of the skull. It is bounded above by the short thick nasals and below by the premaxillae. The narial passage is freely open, maxillo-turbinals not being developed. The palatine is well developed, the pterygoid is small and early fuses with the pterygoid process of the alisphenoid. The tympanic is united with the periotic but not with the squamosal, and forms a large auditorybulla. There are no paroccipital or postglenoid processes. The exoccipital is not perforated by the condylar foramen,—a very exceptional condition.
Fig. 97. Longitudinal section taken rather to the right of the middle line of the skull of a young Indian Elephant(E. Indicus) ×1/8. (Camb. Mus.)8. anterior nares.12. pterygoid.10. periotic.17. nasal.11. palatine.Other numbers as in Fig. 96.
Fig. 97. Longitudinal section taken rather to the right of the middle line of the skull of a young Indian Elephant(E. Indicus) ×1/8. (Camb. Mus.)8. anterior nares.12. pterygoid.10. periotic.17. nasal.11. palatine.Other numbers as in Fig. 96.
Fig. 97. Longitudinal section taken rather to the right of the middle line of the skull of a young Indian Elephant(E. Indicus) ×1/8. (Camb. Mus.)
The mandible has a high ascending portion, is rounded off below and has no angle. The symphysial portion is long, narrow, and spout-like, and the coronoid process is small. The thyro-hyals are ankylosed with the basi-hyal, which is connected with the large forked stylo-hyals by ligament only.
Rodentia.The cranial cavity is depressed, elongated, and rather small, and the cerebral fossa lies entirely in front of the cerebellar fossa. The occipital plane is vertical or directed somewhat backwards, and the supra-occipital does not form much of the roof of the cranium. The paroccipital processes of the exoccipitals are generally of moderate size; in the Capybara (Hydrochaerus), however, they are very long, and are laterally compressed and directed forwards. The parietals are small, and often become completely fused together; there is sometimes a small interparietal. The frontals in most genera have no trace of a postorbital process; in Squirrels, Marmots and Hares, however, one occurs, but in no case does it meet a corresponding process from the zygomatic arch, so the orbit and temporal fossa are completely confluent. In Hares the postorbital process of the frontal is much flattened, and has an irregular margin. The temporal fossa is always small, and inLophiomysis arched over by plates arising respectively from the parietal and jugal; a secondary roof is thus partially developed in a manner unique among mammals, but carried to a great extent in many Chelonia. The nasal bones and cavities are large, attaining their maximum development in the Porcupines (fig. 98, 1). The premaxillae is always very large, and sends back a long process which meets the frontal. The vomer is occasionally found persisting in two separate halves, a feature recalling the arrangement in Sauropsids. In many Rodents there is an enormous vacuity at the base of the maxillary portion of the zygomatic arch. It is sometimes aslarge as the orbit, and attains its maximum development in the Capybara and other Hystricomorpha; in the Marmots, Beavers, and Squirrels (Sciuromorpha), and in the Hares it is undeveloped. InLagostomusthe maxillae bears an upwardly directed plate of bone, shutting off from this vacuity a space which is the true infra-orbital foramen.
Fig. 98. Half front view of the skull of a Porcupine(Hystrix cristata) × ½. (Camb. Mus.)1. nasal.5. premaxillae.2. maxillo-turbinals.6. jugal.3. infra-orbital vacuity.i1. upper incisor.4. maxillae.
Fig. 98. Half front view of the skull of a Porcupine(Hystrix cristata) × ½. (Camb. Mus.)1. nasal.5. premaxillae.2. maxillo-turbinals.6. jugal.3. infra-orbital vacuity.i1. upper incisor.4. maxillae.
Fig. 98. Half front view of the skull of a Porcupine
(Hystrix cristata) × ½. (Camb. Mus.)
The zygomatic arch is always complete, and in many cases the jugal extends back to form part at least of the glenoid surface for articulation with the mandible. InCoelogenysthe jugal and maxillary portion of the zygomatic arch is greatly expanded and roughened, and the maxillary portion encloses a large cavity. The palate in Rodents is narrow, and the space between the incisor and molar teeth passes imperceptibly into the sides of the face. The anterior palatine foramina form long, rather narrow slits in this region. The bony palate between the grinding teeth is sometimes as in the Haresvery short, sometimes as in the Capybara very long. The maxillae extends back beneath the orbit to unite with the squamosal. The pterygoid is always small, but sometimes has a well-marked hamular process which inHystrix,Lagostomus, and some other genera unites with the tympanic bulla. The periotic is large, and fused with the tympanic, which forms a prominent bulla, and is generally drawn out into a tubular meatus. The bulla attains its maximum development inChinchillaandDipus.
The mandible is narrow and rounded in front, the two halves meeting in a long symphysis. The angle is generally drawn out into a long backwardly-projecting process, which is often pointed and directed upwards. In the Hares the angle is rounded. The coronoid process is never large.
There are a number of points in which the skull of the Duplicidentata (Hares and Rabbits) differs from that of other Rodents. (a) The sutures between the basi-occipital and basisphenoid, and between the basisphenoid and presphenoid remain open throughout life. (b) Much of the maxillae forming the side of the face in front of the orbit is fenestrated. (c) The optic foramina are united to form a single hole, much as in birds. (d) The coronoid process is slightly differentiated from the ascending portion of the mandible. The first two of these points have been thought to indicate degradation of the hares and rabbits as compared with higher mammals.
Carnivora[167].It is characteristic of the skull in Carnivora that the glenoid fossa is deep, and the postglenoid process (fig. 75, 23) well developed. The condyle of the mandible is much elongated transversely. The orbit and temporal fossa in the great majority of forms communicate freely, the postorbital bar being incomplete.
Carnivora vera.The axis of the facial portion of the skull is a direct continuation of that of the cranial portion.The cranial cavity though rather depressed is large, and generally long, though in Cats it is comparatively short and wide. The occipital plane is nearly vertical, and the exoccipitals are developed into fairly prominent paroccipital processes. The interparietal is commonly distinct, and the parietals unite in a long sagittal suture, which is often developed into a crest. The nasals (fig. 73, 4) are well developed, especially in Cats, and the nasal processes of the premaxillae do not nearly reach the frontals. A considerable part of the palate is formed by the palatine, and the maxillary portion is pierced by rather long anterior palatine foramina. The pterygoid has a hamular process. The zygomatic arch is strong, especially in Cats. Postorbital processes are developed on the frontal (fig. 73, 10) and jugal, but never form a complete postorbital bar. A carotid canal is well seen in the Ursidae, and to a less extent in the Felidae; in the Canidae there is an alisphenoid canal (fig. 75, 21).
The auditory bulla differs a good deal in the different groups. In the Bears (Ursidae) it is not much inflated, and is most prominent along its inner border; it is not closely connected with the paroccipital process. In the Cats it is very prominent, and its cavity is almost divided by a septum into two parts, the inner of which contains the auditory ossicles. The paroccipital process is closely applied to the bulla. In the Dogs the bulla is intermediate in character between that of the Cats and that of the Bears; it is partially divided by a septum, and is moderately expanded.
The mandible is well developed with a prominent angle (fig. 72, 26), and a large coronoid process. The hyoid consists of a broad basi-hyal, a long many-jointed anterior cornu and short thyro-hyals (fig. 72, 33).
The skull in theCreodontais in most respects allied to that of the Canidae, but presents some ursine affinities. The tympanic bulla is fairly prominent, but has no well-developed septum. The cranial cavity is very small and narrow, thezygomatic arch standing away from it. The temporal fossa is of great size.
In thePinnipediathe cranial cavity is large and rounded. The skull is much compressed in the interorbital region, and in correlation with this compression the ethmo-turbinals are little developed, while the maxillo-turbinals are large. The orbit is large, and the temporal fossa smaller than in the Carnivora vera. In the Walrus (Trichechus) the anterior part of the face is distorted by the development of the huge canines. The Otariidae have an alisphenoid canal. The tympanic bulla is small inOtaria, large in the Phocidae, and flattened in the Walrus. The hyoid is similar to that in Carnivora vera.
Insectivora.The skull varies much in the different members of the order Insectivora, but the following points of agreement are found. The cranial cavity is of small size, and is never much elevated. The facial part of the skull is generally considerably elongated, and the nasals and premaxillae are well developed. The zygomatic arch is usually slender or incomplete, and the coronoid process and angle of the mandible are commonly prominent.
In some Insectivora, such asGaleopithecus,Tupaia, andMacroscelides, the skull shows a higher type of structure than is met with in most members of the order. In these genera the cranial cavity is comparatively large, and the occipital plane is nearly vertical. The zygomatic arch is fairly strong, and the frontal and jugal give rise to postorbital processes which nearly or quite (Tupaia) meet. The tympanic bulla is well developed, and produced into a tubular auditory meatus, this being specially well marked inMacroscelides.
In the other Insectivora the cranial cavity is of smaller comparative size, and the orbit and temporal fossa are completely confluent, often without any trace of a postorbital bar. The occipital plane commonly slopes forwards. In the Hedgehogs (Erinaceidae) and Centetidae the tympanic is very slightly developed, forming a small ring. The zygomaticarch of Hedgehogs andGymnurais very slender, the jugal being but little developed and the squamosal and maxillae meeting one another; in the Centetidae the jugal is absent and the arch is incomplete.
The Moles (Talpidae) have an elongated, depressed and rounded skull with a very slender zygomatic arch formed by the squamosal and maxillae. The nasals are fused together, and the mesethmoid is ossified very far forwards. In the Shrews (Soricidae) there is no zygomatic arch; the tympanic is ring-like, and the angle of the mandible is very prominent. The hyoid has a transversely extended basi-hyal, a long anterior cornu with three ossifications, and thyro-hyals which are sometimes fused to the basi-hyal.
Chiroptera.In the frugivorous Flying Foxes (Pteropidae) the skull is elongated, and the cranial cavity is large and arched, though considerably contracted in front. There are commonly strong sagittal and supra-orbital crests. The parietals take a great part in the formation of the walls of the cranial cavity, the supra-occipital and frontals being small. The frontal is drawn out into a long postorbital process, but the zygomatic arch, which is slender, and formed mainly by the squamosal and maxillae, gives rise to only a small postorbital process, so that the orbit and temporal fossa are confluent. There is no alisphenoid canal, and the tympanics are very slightly connected with the rest of the skull. The mandible has a large coronoid process, a rounded angle, and a transversely expanded condyle.
In Insectivorous Bats the skull is generally shorter and broader than in the Pteropidae. The cranial cavity is large and rounded, and has thin smooth walls. The zygomatic arch is slender, and postorbital processes are not generally well developed. The premaxillae is generally small, sometimes absent. The tympanics are ring-like and are not connected with the surrounding bones. The angle of the mandible is distinct. The hyoid in most respects resembles that of the Insectivora.
Primates.The characters of the skull differ greatly in the two suborders of Primates, the Anthropoidea and the Lemuroidea.
In theLemuroideathe general relative proportions of the cranium and face are much as in most lower mammals, and the occipital plane forms nearly a right angle with the basicranial axis. The postorbital processes of the frontals are commonly continued as a pair of ridges crossing the roof of the cranium and meeting the occipital crest. Though the postorbital bar is complete, the orbit and temporal fossa communicate freely below it. The lachrymal canal opens outside the orbit, and the lachrymal forms a considerable part of the side of the face. The tympanic is developed into a large bulla. The hyoid apparatus much resembles that of the Dog.
In theAnthropoideathe skull differs greatly from that in the Lemuroidea. The cranial portion of the skull is very large as compared with the facial portion, though the comparative development varies, some monkeys, such as the baboons (Cynocephali) having the facial portion relatively large. The comparative size of the jaws does not vary inversely with the general development of the animal, some of the Cercopithecidae having comparatively larger jaws than some of the Cebidae. The great size of the cranial part of the skull is mainly due to the immense development of the cerebral fossa, which commonly completely overlaps the olfactory fossa in front, and the cerebellar fossa behind. This development also has the effect of making the ethmoidal and occipital planes lie, not at right angles to the basicranial axis, but almost in the same straight line with it. This is, however, not always the case, as the Howling Monkey (Mycetes) and also some of the very highest monkeys, the Gibbons (Hylobates), have the occipital plane nearly vertical to the basicranial axis. In adult Man the basi-occipital, exoccipitals and supra-occipital coalesce, forming the so-called occipital bone; while the basisphenoid, presphenoid, alisphenoids, orbitosphenoids and pterygoids formthe sphenoid bone. The roof of the skull is partly formed by the large supra-occipital and frontals, but mainly by the parietals (fig. 99, 1), which in Man are of enormous extent.