39o 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 premaxUlse. 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 maxilla?, 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 bounded 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 



