HYRACOTHERIUM. 329 



{see Fig. 98). Hence, I am inclined to think, that the Hip- 

 parion, whose immediate ancestors were digitigrade animals, 

 used the second and fourth digits, at least of its fore feet, to 

 some extent at fast paces ; if not in slow movement. 



Still earlier, we find in the middle Eocene age, the 

 Paloplotherium, which resembled the Anchitherium and 

 Hipparion in its feet {see Fig. 194); and the horse tribe, 

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

 than that of its successors. The foot shown in Fig. 194 is 

 that of a small variety of Paloplotherium. Among the 

 Eocene deposits in America is found the Orohippus agilis f 

 (Fig. 195), 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. 196), which had three toes on its hind 

 feet and four toes on its front feet, 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. Although the Hyraco- 

 therium had four toes on its front feet, it may be considered 



* The teeth of the Paloplotherium have a plentiful ■iW'^^Xy oi cricsta petrosa 

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

 cement is the outer layer of the teeth. In youth, it forms a thin covering to 

 the enamel on the outside ; and a thick one in 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" in the teeth its characteristic dark appearance. The front teeth 

 of the horse become white with age from the wearing away of the cement. 



\ ' Opos, limit ; iTTTros, horse. 



