THE EVOLUTION OF LIFE 



353 



Equus: Qua- 

 ternary and 

 Recent. 



Pliohippus : 

 Pliocene. 



Protohippus : 

 Lower Plio- 

 cene. 



Miohippus : 

 Miocene. 



Mesohippus : 

 Lower Mio- 



Orohippus : 

 Eocene. 



FIG. 242. Feet and teeth in evolution of the horse; a, bones of fore- 

 foot; b, bones of hind foot; c, radius and ulna; d, tibia and fibula; e, roots 

 of a tooth; / and g, crowns of upper and lower polar teeth. (From 

 Romanes after Marsh.) 



