GENERAL LAWS OF ANIMAL STRUCTURE. 259 

 " * e d e f g 



9 



Equus: Qua- 

 ternary and 

 Recent. 



Fliohippus : 

 Pliocene. 



Protohippus : 

 Lower Plio- 

 cene. 



Fig. 165. — Diagram illustrating gradual changes in the horse family : 

 Throughout a is fore foot ; b, hind foot ; c, forearm ; d, shank ; e, molar 

 on side view ; f and g, grinding surface of upper and lower molars. 

 (After Marsh.) 



