CENOZOIC ERA. AGE OF MAMMALS. 



a b c d e f 



Pliohippus: 

 Pliocene. 



Protohippus 

 Lower Plio- 



Miohlppus 

 Miocene. 



Mesohippus 

 Lower Mio- 



Orohippus 

 Eocene. 



FIG. 345. Diagram illustrating gradual changes in the horse family. Throughout, 

 rt is fore-foot ; ft, hind-foot ; c, fore-arm ; </, shank ; e, molar on side-view ; 

 / and g, grinding surface of upper and lower molars. (After Marsh.) 



