HISTORY OF THE PERISSODACTYLA 



337 



identical, for fTrigonias was an undoubted rhinoceros. Such 

 an identification, however, would be a great mistake, for the 



Fig. 176. — Skull of t Trigonias osbomi, lower White River. (After Hatcher.) 



differences, though not striking, are very important. In the 

 upper jaw the first or anterior incisor had already assumed the 

 characteristic trenchant, chisel-Uke shape, but two other 

 incisors were present also, thus bringing the number up to 

 the original three, common to all early perissodactyls ; even 

 more interesting is the presence of a small upper canine. The 

 lower jaw likewise had 

 three incisors on each 

 side, the first and third 

 small, the second en- 

 larged and tusk-Uke, but 

 the canine had already 

 been suppressed, and 

 thus the dental formula 

 was : if, c^, p|, m|, X 2 = 

 42, or 14 more than the 

 formula of the existing 

 African species. The premolars were smaller and less complex 

 than the molars. 



From this ancient genus may readily be inferred the steps 



Fig. 177. — Anterior end of right upper jaw ol 

 t Trigonias osbomi (after Lucas), c, canine. 

 i S, external incisor, i 2, middle incisor, i 1 , 

 first incisor. 



