350 LAND MAMMALS IN THE WESTERN HEMISPHEEE 



them, family or subfamily, much the most prolific in diver- 

 gent forms was that of the true rhinoceroses (Rhinocerotidae) 

 of which seven or more phyla have been distinguished, three 

 of them surviving to the present time. Only in this series 

 were horns frequently present, the brief experiment, as it 

 might be called, of the Bridger genus ]Colonoceras, being the 

 only known instances of horns among the fhyracodonts, and 

 the famynodonts were all hornless. In making the comparison 

 as to degree of ramification among the three series, it should 

 be borne in mind that the true rhinoceroses were the only 

 long-lived group, the other two dying out before or at the end 

 of the White River stage. Within the series or family of the 

 true rhinoceroses, there was no great divergence of type, and 

 all the members were much alike, heavy and slow animals, 

 but with very great variety in the details of structure. Take, 

 for instance, the matter of horns ; we find both hornless and 

 horned genera, the former preceding the latter in time, but, 

 so far as North America is concerned, continuing in associa- 

 tion with them till the end. Among the horned genera, the 

 horn may be single, double in a transverse pair {\Dicera- 

 therium) or arranged one behind the other in the median line 

 of the head {Dicer orhinus, Opsiceros, etc.). The single horn 

 may be on the nose or the forehead ; if on the nose, it may be 

 on the upper side of the nasal bones (Rhinoceros) or on 

 the extreme tip and jJointing obhquely forward (1[Teleoceras). 

 The single frontal horn was much less common, but in the ex- 

 traordinary '\ElasmotHerium, of the European and Siberian 

 Pleistocene, the horn was of gigantic size and the surface for 

 its attachment an enormous, dome-like boss on the forehead. 



All three of the series had their most ancient known repre- 

 sentatives in North America, and it seems probable, though 

 by no means certain, that they all originated here by diver- 

 gence from a common stock, which was represented more or 

 less closely by the genus ^Hyrachyus of the Bridger and Wind 

 River stages of the Eocene. However that may be, true rhinoc- 



