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 in Rhinoceros. 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 as Elasmotherium and 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 (perhaps only 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.
In Tapirus the 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 genus Equus there 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 in Rhinoceros; 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 the Subungulata, the lachrymal and jugal do not form any 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 in Typotherium. The mandible has a rounded angle and a coronoid process of moderate size. In Typotherium the 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 in Phenacodus being exceptionally small. The orbit is completely confluent with the temporal fossa.
Hyracoidea. The skull of Procavia resembles 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 in Procavia the frontal often forms part of the process. The alisphenoid canal, and postglenoid and paroccipital processes are well developed. The tympanic bulla is large and the periotic 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.