468 



THE CHANGING GENERATIONS 



crowned, with cusps and valleys instead of a grinding surface, and the 

 posterior premolars were just beginning to become molarlike. Without 

 the indications given by the later fossils it is doubtful if Eohippus would 

 have been recognized as a horse; and there can be no doubt that it is 

 closely related to other similar contemporary mammals which were 

 ancestral to rhinoceroses, tapirs, and the other perissodactyl stocks. 



Mesohippus, an Oligocene genus, shows some advance over Eohippus. 

 It was larger — about the size of a collie dog — and had lost all but a vestige 

 of the outer toe of the front feet, so that both feet were three-toed, though 



Fig. 28.33. A group of Oligocene horses, genus Mesohippus. 

 History Museum.) 



(Courtesy Chicago Natural 



still pawlike, with all the toes touching the ground. The radius and ulna 

 and the tibia and fibula showed incipient fusion. The skull and teeth were 

 not much changed; the face had lengthened slightly, though the orbit 

 remained open behind, and the three posterior premolars had become 

 quite like the permanent molars, which were still of the short-crowned 

 browsing type. 



Merychippus, of the Miocene, is a long step forward from the forest- 

 dwelling Mesohippus. It lived at a time when forests were giving way to 

 grasslands, and its structure shows that Merychippus had taken to the 

 plains and to a diet of grass. Its height had increased to 3 feet. The legs 

 were stronger, with the two bones of the lower leg firmly fused together 

 in the adult though still separate in the colt. There were still three toes, 



