58 THE HORSE. 



ing from the evidence at present available, this was 

 the locality in which they first made their appearance. 

 In the Eocene formations of the Rocky Mountains 

 are found the remains of numerous modifications of 

 the primitive Perissodactyle type, from w^hich the 

 rhinoceroses may have originated. In the Lower 

 Miocene a form called Hyracodon by Leidy already 

 presented many of the characteristics of the family, 

 though, especially as regards the dentition, still in 

 a very generalized condition. It had, however, 

 already lost the fifth toe of the fore-foot. The next 

 stage of specialization is represented by Aceratheri- 

 um and Aphelops, found in the Miocene of Europe 

 and America, which still, like the last, show no sign 

 of having possessed a nasal horn. The former 

 differs from the existing species, and also from Hy- 

 racodon, in having four toes on the anterior limb 

 instead of only three. At the same period forms 

 occurred (Dicer atherium. Marsh) which show a pair 

 of lateral tubercles on the nasal bones apparently 

 supporting horns side by side. These, however, soon 

 disappeared and gave way in the Old World to species 

 with one or two horns in the median line, a stage of 

 development which apparently was never reached in 

 America. In the Pliocene and Pleistocene of Europe 

 and Asia numerous rhinoceros remains have been 

 found, all more or less nearly related to the existing 



