HISTORY OF THE PERISSODACTYLA 



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 Cm no pus tridactylus) ; middle White River 



stags. (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- 

 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- 

 cene and subsequent genera. 



As already mentioned, there was much 

 variation in size among the species, but 

 none was as large as those of the Miocene and Phocene 

 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 Wliite River substage (-fCceno- 

 pus occidentalis) was an animal nearly equalling in size the 

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



Fig. 175. — Second up- 

 per molar, left side, 

 of t Canopus, show- 

 ing the masticating 

 surface. 



