332 LAND MAMMALS IN THE WESTERN HEMISPHERE 



of development ; there was a single trenchant upper incisor 

 on each side, a procumbent lower tusk and between the tusks 

 a pair of small incisors ; the other incisors and the canines 

 were already lost. One genus {\Peraceras) had lost all the 

 upper front teeth. The grinding teeth had the same character 

 as in the existing species, but were somewhat simpler, owing 

 to less development of the accessory spurs. In the more pro- 

 gressive types the teeth were rather high-crowned, though in 

 none were they actually hypsodont ; while the persistent 

 ancient genera had teeth with much lower crowns. 



Aside from the differences in the skull, which are obviously 

 to be correlated with the absence or very small size of the 

 horn, the skeleton in these Pliocene genera differed but little 

 from the type common to the existing rhinoceroses, and in 

 all the species the feet were three-toed. In short, the denti- 

 tion and skeleton, except the skull, had already attained to 

 substantially the modern conditions. While the Old World at 

 that time had both horned and hornless rhinoceroses in abun- 

 dance, none of the genera with large and fully developed horns 

 ever migrated to the western hemisphere. This is the more 

 remarkable in that the great f Woolly Rhinoceros {Opsiceros 

 ]antiquitatis) of the Pleistocene, which had two very large 

 horns, inhabited Siberia with the fMammoth {Elephas fprimi- 

 genius). The latter extended its range through Alaska and 

 the northern United States, but the rhinoceros, for some 

 unknown reason, did not accompany it in its eastward 

 wanderings. 



The rhinoceroses of the upper Miocene did not differ suffi- 

 ciently from those of the lower Pliocene to call for particular 

 attention. Needless to say, there were differences between 

 the species of the two epochs, but in such a sketch as this only 

 the broader and more obvious changes can be taken into account. 

 Even in the middle Miocene the only feature which calls for 

 notice was the first appearance in North America of the Old 

 World genus '\Teleoceras, which became so abundant in the 



