HYRA CO THERIUM, 



2QS 



Paloplotherium, which resembled the Anchitherium and 

 Hipparion in its feet (Fig. 387) ; and the horse tribe, 

 generally, in its teeth.* Its principal digit was much 

 weaker than that of its successors. The foot shown in 

 Fig. 387 is that of a small variety of Paloplotherium. 

 Among the Eocene deposits in America is found the 

 Orohippus agilis\ (Fig. 388), which, as the name given to 

 it by Professor Marsh implies, may be taken as the limit to 

 which we can fairly trace the horse in his character of a 

 one-toed animal. Going further back in the Eocene period, 

 we meet with the Hyracotherium (Fig. 389), which had 

 three toes on its hind feet and four toes on its front feet, 



Fig. 389.— HYRACOTHERimi (Jth leal heights 



and which had hoofs on all of them. It was about 15 

 inches high, and was closely akin to the tapir, which has 

 maintained its peculiarities of conformation, up to the present 

 day, almost without change, during that vast period of time. 



* The teeth of the Paloplotherium have a plentiful supply of crusta petrosa 

 (cement), and thus resemble those of the hoise. I may explain that the cement is 

 the outer layer of the teeth. In youth, it forms a thm covering to the enamel on 

 the outside ; and a thick one m the depressions which all the teeth, with the 

 exception of the tushes, have on their crowns. The cement becomes quickly 

 stained by the action of the food and gives the "mark" m the teeth its charac- 

 teristic dark appearance. The front teeth of the horse become white with age 

 from the wearing away of the cement. 



t ^Opos-, limit ; tTTTroy, horse. 



