700 



COLLEGE ZOOLOGY 



(i) Hyracotherium and Eohippus (Fig. 553). These animals 

 lived during the lower Eocene Period. Only the skull of Hyraco- 

 therium has been discovered, but this shows it to be the most 

 primitive stage known. Eohippus was named from remains 

 found in the Lower Eocene of Wyoming and New Mexico; its 

 forefeet have four complete toes and the splint of the fifth, and 



F IG - 553- — Restoration of the four-toed horse, the oldest known ancestor of 

 the modern horse; only 16 inches high. (From Matthew, after Knight.) 



the hind feet have three complete toes and the splint of the 

 fifth. 



(2) Protorohippus and Orohippus. These forms lived during 

 the Middle Eocene Period and were about as large as a small dog. 

 The feet are similar to those of Eohippus, except that the splint of 

 the fifth digit has entirely disappeared. Remains of an animal 

 called Epihippus are recorded from the Upper Eocene. 



(3) Mesohippus. This animal belongs to the Oligocene Period, 

 and reached the size of a sheep. Its fore feet possess three 



