HISTORY OF THE PERISSODACTYLA 351 



eroses flourished exceedingly in the Old World from the 

 upper Oligocene to the Pleistocene, the events of the latter 

 epoch restricting them to their present range. The signifi- 

 cance of the American genera for the ancestry of the modern 

 types can be found only in the most ancient forms, '\Tri- 

 gonias and "fCcenopus ; the subsequent development which led 

 up to the existing species of Asia and Africa went on entirely 

 in the eastern hemisphere. The fhyracodont subfamily 

 had no known representatives outside of North America, 

 but the famynodonts sent out emigrants, which appeared 

 for a brief time in the Oligocene of Europe. 



In the varied history of the rhinoceroses, the principles 

 of evolutionary change which may be deduced from the recorded 

 development of the horses, tapirs and ftitanotheres are found 

 to be applicable. 



(1) There was the same gradual increase in size from the 

 earlier to the later geological stages. Not that all the phyla 

 kept equal pace in this respect, and even within the same 

 phylum it was the rule rather than the exception to find 

 larger and smaller contemporary species. 



(2) In all of the early forms, up to the middle Miocene, 

 the teeth were low-crowned ; after that time there was a 

 decided increase in the height of the teeth, though only in 

 ^Elasmotherium was the fully hypsodont, cement-covered 

 crown attained. In the existing African Broad-Lipped Rhi- 

 noceros (Opsiceros simus), which is a grazing animal, the high, 

 cement-covered teeth may also fairly be called hypsodont. 



(3) In all of the lines, as in the other perissodactyl families, 

 the premolars gradually took on the pattern of the molars ; 

 only in the jamynodonts were the premolars notably reduced 

 in number and size. 



(4) The three different modes of development of the anterior 

 teeth, exemplified by the true rhinoceroses, the fhyracodonts 

 and famynodonts respectively, need not be recapitulated 

 here. It is sufficient to call attention to the fact that the three 



