390 UNGULATA 
forms part of the posterior and inferior boundary of the orbit, an 
arrangement not met with in other mammals. The closure of the 
orbit behind distinguishes the skull of the Horse from that of the 
Rhinoceros and Tapir, and also from all of the Perissodactyles of 
the Eocene period. In front of the cerebral cavity, the great 
tubular nasal cavities are provided with well-developed turbinal 
bones, and are roofed over by very large nasals, broad behind, and 
ending in front in a narrow decurved point. The opening of the 
anterior nares is prolonged backwards on each side of the face 
between the nasals and the elongated slender premaxille. The 
latter expand in front, and are curved downwards to form the semi- 
circular alveolar border supporting the large incisor teeth. The 
palate is narrow in the interval between the incisor and cheek- 
teeth, in which are situated the large anterior palatine foramina. 
Between the cheek-teeth it is broader, and it ends posteriorly in a 
rounded excavated border opposite the hinder edge of the penulti- 
mate molar. It is mainly formed by the maxille, as the palatines 
are very narrow. The pterygoids are delicate slender slips of bone 
attached to the hinder border of the palatines, and supported 
externally by, and generally ankylosed to, the rough pterygoid 
plates of the alisphenoid, with no pterygoid fossa between. They 
slope very obliquely forwards, and end in curved, compressed, 
hamular processes. There is a distinct alisphenoid canal for the 
passage of the internal maxillary or main branch of the external 
carotid artery. The base of the cranium is long and narrow; the 
alisphenoid is very obliquely perforated by the foramen rotundum, 
but the foramen ovale is confluent with the large foramen lacerum 
medium behind. The glenoid surface for the articulation of the 
mandible is greatly extended transversely, concave from side to 
side, convex from before backwards in front, and hollow behind, and 
is hounded posteriorly at its inner part by a prominent postglenoid 
process. The squamosal enters considerably into the formation of 
the temporal fossa, and, besides sending the zygomatic process for- 
wards, it sends down behind the meatus auditorius a post-tympanic 
process which aids to hold in place the otherwise loose tympano- 
periotic bone. Behind this the exoccipital gives off a very long 
paroccipital process. The periotic and tympanic are ankylosed 
together, but not with the squamosal. The former has a wide but 
shallow floccular fossa on its inner side, and sends backwards a 
considerable “ pars mastoidea,” which appears on the outer surface 
of the skull between the post-tympanic process of the squamosal and 
the exoccipital. The tympanic forms a tubular meatus auditorius 
externus directed outwards and slightly backwards. It is not 
dilated into a distinct bulla, but ends in front in a pointed styliform 
process ; and completely embraces the truncated cylindrical tym- 
panohyal, which is of great size, in correspondence with the large 
