54 



THE HOESE 



tained many more species and was much more widely 

 distributed than at present. As already indicated, our 

 knowledge of them is as yet but fragmentary, though 

 constantly augmenting, especially by discoveries made 

 in the Tertiary deposits of North America, a region 

 from which they all died out long ago, though, judg- 

 ing from the evidence at present available, this was 

 the locality in which they first made their appearance. 

 In the Eocene formations of the Eocky Mountains are 

 found the remains of numerous modifications of the 

 primitive Perissodactyle type, from which the rhinoce- 

 roses may have originated. In the Lower Miocene a 

 form called Ilyracodon by Leidy already presented many 

 of the characteristics of the family, though, especially as 

 regards the dentition, still in a very generalised condi- 

 tion. It had, however, already lost the fifth toe of the fore- 

 foot. • The next stage of specialisation is represented by 

 Acerathermm 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 Hyracodon, in 

 having four toes on the anterior limb instead of only 

 three. At the same period forms occurred (Dicerathe- 

 rium. 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 



