542 



CENOZOIC ERA— AGE OF MAMMALS. 



In a similar way Cope lias traced the line of descent of the camel 

 a b c d e f 



Equus : Quaternary and 

 Eecent. 



Pliohippus: Pliocene. 



Protohippus; Lower Pliocene. 



Itiohippus: Miocene. 



Mesohippus: Lower Miocene. 



Orohippus: Eocene. 



Fig. 922.— Diagram illustrating Gradual Changes in the Horse Family. Throughout a is fore-foot; 

 b, hind-foot: c, fore-arm: d, shank; e, molar on side view; /and g, grinding surface of upper 

 and lower molars. (After Marsh.) 



