250 THE HORSE AND ITS RELATIVES 



In North America the immediate precursor of 

 the modern Equus is the genus Pliohippus^ whose 

 remains occur in deposits belonging to the Lower 

 Pliocene and to the antecedent Miocene epoch. 

 Whether this was a completely single-toed animal, 

 or whether there were small remnants of the lateral 

 toes, appears uncertain. If these were present 

 they must, however, have been extremely small 

 and quite functionless, as the splint-bones are 

 scarcely larger than in the modern horse. ^ The 

 cheek-teeth are larger than those of the under- 

 mentioned Protohippus, but owing to the relative 

 shallowness of the jaws, as compared with those of 

 EquuSy their crowns were still more sharply curved. 

 Pliohippus, as typified by P. pernix of the Loup 

 Fork beds of Nebraska, was the largest of the con- 

 temporary horses, standing about 12 hands at 

 the withers, and thus equalling a good-sized pony. 

 The rancre of the pfenus included the Western 

 United States, especially Nebraska and Oregon. 

 Not improbably this or a closely allied form was 

 the direct ancestor of Equus. 



It has likewise been suggested that Pliohippus 

 gave rise to the remarkable Hippidium {^Hippidion) 

 and Onokippidium of the Pleistocene deposits of 

 South America : this is considered problematical 

 by Prof. Lull, although more favourably received 



^ See R. S. Lull, "The Evolution of the Horse Yaxmly" Amer. /. 

 Science, ser. 4, vol. iii. p. 478, 1907. 



