IV.] 



PERISSODACTYLES. 



I6 7 



(Mesohippus) bairdi of the White River Oligocene of the United 

 States, the incisor teeth lack the infoldings of the summit of the 

 crown characteristic of the horses, and the lateral digits are 

 relatively large ; the whole size of the creature being comparable 



FIG. 37. LEFT UPPER CHEEK-TEETH OF Anchitherium. 



to that of a Newfoundland dog. On the other hand, in the typical 

 A. aurelianense, from the European Miocene, the summits of the 

 incisors were infolded, as in the horses. In spite of this resem- 

 blance, Professor Scott, from the structure of the limbs, is of 

 opinion that the latter species was not an ancestor of the modern 

 horses, but that this position was occupied by A. bairdi. 



Restricting the term Equidce to those members of the suborder 

 in which the crowns of the cheek-teeth are very tall (Jiypsodonf), 

 with complicated infoldings of their enamel, and the hollows thus 

 formed completely filled with cement, we have in the lower 

 Pliocene of North America the three-toed genus Protohippus, 



FlG. 38. RIGHT UPPER MOLAR TOOTH OF A HORSE (Equus). 



distinguished from the modern horses by the shorter crowns of the 

 cheek-teeth. The widely-spread genus Hipparion differs in having 

 the anterior inner column of the upper molars completely 



