HISTORY OF THE PERISSODACTYLA 



339 



poUex, or first of the original five, almost always the first to 

 disappear, had been suppressed, the third or median digit was 

 already the largest of the series, both in length and breadth ; 

 the second and fourth, some- 

 what shorter together made a 

 symmetrical pair, while the 

 fifth, though much the most 

 slender of all, was still func- 

 tional and had retained all of 

 its parts. In the hind foot 

 the digits had been reduced 

 to three. This arrangement, 

 four toes in the manus and 

 three in the pes, is the same as 

 is found in the existing tapirs 

 and in the Eocene perissodac- 

 tyls generally, with only two or 

 three known exceptions. In 

 the Oligocene, on the other 

 hand, all the genera except the 

 ftitanotheres, tapirs, flophiodonts and famynodonts were tri- 

 dactyl both before and behind. 



With \Trigonias the definitely known history of the true 

 rhinoceroses breaks off abruptly, and it is possible that that 

 genus was an immigrant, though it is perhaps more likely that 

 its ancestors existed in the upper and middle Eocene (Uinta 

 and Bridger stages) of North America. Some fragmentary 

 specimens from the Uinta beds, too imperfect for any definitive 

 identification, are an encouragement to hope that the fore- 

 runner and direct ancestor of ^Trigonias may yet be dis- 

 covered in that formation. It is also quite possible that one of 

 the larger species of the genus fHyrachyus, so abundant in the 

 Bridger and going back to the Wind River, may take its 

 place in the same series. 



Fig. 179. — Left manus of t Trigonias os- 

 homi. (After Hatcher.) 



