Evolution of the Earth, 



99 



Equus : Qua- 

 ternary and 

 Recent. 



Pliohippus ; 

 Pliocene. 



Protohippus : 

 Lower Plio- 

 cene. 



Miohippus ; 

 Miocene. 



Mesohippus : 

 Lower Mio- 

 cene. 



Orohippus 

 Eocene. 



Diagram illustrating the gradual evolution of the horse-family (after Pro- 

 fessor Marsh), — kindly furnished by Messrs. D. Appleton & Co. Throughout 

 the series, a indicates fore-foot ; b, hind-foot ; c, f ore-arm ; d, shank ; e, molar, 

 side-view; /and g, grinding surface of upper and lower molars. The Eohip- 

 pus of the Lower Eocene, and the Phenacodus (Cope), are not represented in 

 this diagram. 



