XL] PERISSODACTYLA. 189 



completely ankylosed to, the lateral extremities of the 

 basihyal. 



Each ramus of the mandible has a long, straight, compressed 

 horizontal portion, gradually narrowing towards the symphysis, 

 where it expands laterally to form with the ankylosed opposite 

 ramus the wide semicircular, shallow, alveolar border for 

 the incisor teeth. The region of the angle is expanded and 

 compressed, with a thickened rounded border without any 

 process. The condyle is greatly elevated above the alveolar 

 border ; its articular surface is very wide transversely, and 

 narrow and convex from before backwards. The coronoid 

 process is slender, straight, and inclined backwards. 



The skull of the Rhinoceros resembles that of the Horse 

 in many essential features, but the occipital region is of 

 much greater extent vertically, the form of the cranial cavity 

 being concealed externally by large occipito-parietal air- 

 cells. There is no postorbital process to the frontal, so 

 that the orbit is not divided from the temporal fossa. 

 There is a conspicuous, rough, antorbital projection on the 

 lacrymal bone just in front of the lacrymal foramen 

 The nasals are very large and strong, early ankylosed to 

 gether, arched from before backwards, and pointed anteriorly. 

 The most elevated part of their upper surface is roughened, 

 and supports the great median horn which characterises 

 the genus. In some species a posterior rough, but less 

 elevated, surface indicates the attachment of a second 

 horn. In some of the extinct species the mesethmoid 

 cartilage was ossified nearly as far forwards as the extremity 

 of the nasals, which is not the case with any existing 

 species. The premaxillae are very small, and do not extend 

 anteriorly beyond the level of the front end of the nasals. 

 The hinder border of the palate is deeply excavated, the 



