HISTORY OF THE PEKISSODACTYLA 



335 



making the incisor formula | ; the third incisor and the canines 

 of both jaws were already lost. The assumption of the molar- 



FiG. 174. — Skull of t hornless rhinoceros (t Ccenopus tridactylv^) ; middle White River 



stage. (After Osborn.) 



pattern by the premolars varied much in degree of complete- 

 ness in the different species ; the upper molars, while having all 

 the essentials of the rhinocerotic-plan of struc- 



I ture, had a much less complex appearance 



\ than in the Recent genera, because of the ab- 

 sence of the accessory spurs; and all the 

 grinding teeth were very low-crowned, in 

 strong contrast to the high-crowned (yet not 

 properly hypsodont) teeth of the middle Mio- ^°- i75.-Secondup- 



per molar, left side, 



cene and subsequent genera. of t Ccenopus, show- 



As aheady mentioned, there was much ^^f *Je. °'^'*'"''""^ 

 variation in size among the species, but 

 none was as large as those of the Miocene and PUocene 

 genera, not to mention the enormous animals of the Pleis- 

 tocene and Recent epochs in the Old World. The com- 

 moner species of the middle White River substage (fCosno- 

 pus ocddentalis) was an animal nearly equalUng in size the 

 American Tapir {T. terrestris) and quite hke that species in 



