i68 THE SKULL. [chap. 



The nasal bones are short, broad behind, pointed in front, 

 much elevated, and supported by a tolerably well ossified 

 mesethmoid, which spreads out laterally at its upper end. 1 

 The inferior and lateral margins of the great narial apertures 

 are entirely formed by the maxillae, which extend up to meet 

 the nasals, neither frontals nor premaxillae taking any share 

 in them. The ethmoturbinals are small, while the maxillo- 

 turbinals, on the other hand, are very extensive, though 

 their plications are comparatively simple. A conspicuous 

 feature in the upper part of the face is a groove, which 

 extends backwards on the side of the dilated hinder end 

 of the nasal bone, and curves inwards to form a rounded 

 depression over the naso-frontal suture. The form and size 

 of this depression vary in different species. It lodges an air 

 sinus, with cartilaginous walls extending upwards from the 

 nasal chamber. In front of the nares, the rostrum formed 

 by the maxillae with the premaxillae in front, is produced, 

 compressed anteriorly, and curved downwards. 



The base of the cranium resembles generally that of the 

 other Perissodadyla. There is an alisphenoid canal, and 

 large postglenoid and post-tympanic processes; the latter 

 joins the paroccipital process of the exoccipital, but above 

 their point of union a narrow slip of the mastoid appears 

 on the surface of the skull. The periotic is not ankylosed 

 to the squamosal or to the tympanic, which is exceedingly 

 rudimentary, forming a small irregular floor to the tympanic 

 cavity, with an oval lip for the attachment of the membrana 

 tympani, and always becomes detached in macerated skulls. 



The mandible is chiefly noticeable for the great rounded 

 incurved posterior projection of the angle. 



1 In one species ( T. Bairdii) the ossification of the mesethmoid extends 

 far in advance of the nasal bones, and is clasped and supported below 

 by ascending plates from the maxillae. 



