Equus : Qua- 

 ternary and 

 Recent. 



Pliohippus : 

 Pliocene. 



Protohippus : 

 Lower Plio- 

 cene. 



Miohippus : 

 Miocene. 



Mesohippus ; 

 Lower Mio- 

 cene. 



Orohippus : 

 Eocene. 



DIAGRAM ILLUSTRATING THE EVOLUTION OF THE 

 HORSE. 



Throughout a. is fore-foot ; i, hind-foot ; c, fore-arm ; d, shank : 



e, molar on side-view ; y and ^, grinding surface of 



upper and lower molars. (After Marsh.) 



