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 hy Messrs. 1). Ajipleton & Co. Throughout 

 the series, a indicates fore-foot ; /', hind-foot ; c, fore-arm ; (/, shank ; r, 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. 



