338 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



by which the pecuUar characters of the anterior teeth in the 

 true rhinoceroses were attained. The first stage was undoubt- 

 edly an animal in 

 which, as in all 

 other Eocene peris- 

 sodactyls, there 

 were three well-de- 

 veloped incisors on 

 each side of both 

 jaws, 12 in aU, and 

 moderately promi- 

 nent canine tusks; 

 all these teeth were 

 erect. The second 

 stage was the en- 

 largement of . the 

 first upper and sec- 

 ond lower incisors, 

 the latter becoming 

 less erect and begin- 

 ning to assume the recumbent position ; at the same time the 

 other incisors and the canines were reduced in size and were so 

 little used that they lost their functional importance. The 

 third stage, in which the first and second lower incisors were 

 horizontal and pointed directly forward, and the first upper and 

 second lower teeth were still further enlarged, the non-func- 

 tional teeth reduced in size and the lower canine suppressed, 

 was realized in the genus ^Trigonias. There were thus but 

 two hypothetical stages between this lower White Region genus 

 and the tapir-like forms of the middle Eocene, so far, at least, 

 as the anterior teeth are concerned. 



The skeleton of \Tngonias was, on the whole, very much 

 like that of the succeeding genus, ^Cceno'pus, of the middle 

 substage of the White River, but with the important exception 

 that the front foot had four digits instead of three. The 



A Ti. 



Fig. 178. — Anterior end of left upperjaw of t Ccsnopus, A, 

 adult ; B, immatinre animal (after Osborn) . II, first 

 incisor ; 1 2, second incisor ; C, canine. 



