UNGULATA. 333 



Russia, is specialized not only in the development of a great 

 horn, but in the deepened (hypsodont) prismatic form of 

 its grinding teeth. 



The crowns of the molars and premolars in all the 

 rhinoceroses are somewhat deeper than those of the other 

 perissodactyls already noticed ; but, except in the aberrant 

 Elasmotherium just mentioned, the hollows between the ridges 

 are not filled with cement. The hindermost upper molar is 

 reduced to a triangular form by the partial atrophy of its 

 posterior ridge ; while the hindermost lower molar consists 

 of never more than two crescents, like those in front of it. The 



a.cr 



FIG. 191. 



Second Bight Upper Molar of Rhinoceros, much reduced, a.cr., crochet; ap. , 

 parastyle;, cr., anticrochet ; cr*., crista ; me. , metacoue ; pa., paracone. 

 (From Osborn, after De Blainville.) 



upper molars (fig. 191), except in some of the earliest forms, 

 are more or less complicated by the growth forwards of a 

 prominence or " crochet " (a.cr.) from the posterior ridge, and 

 by another similar prominence ("crista" or "combing plate," 

 cr*) growing from the outer wall into the median valley; and 

 these two points sometimes meet. A third prominence or 

 "anticrochet" (cr.) also sometimes projects backwards from the 

 anterior ridge to meet the others. There is always a diastema 

 in front of the premolars; while the canines and incisors are 

 very variable in development, sometimes even absent. The 

 orbital cavity is never separated by bone from the temporal 

 fossa. There are always three functional digits on each foot. 



