72 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Then came in succession Orohippus, of the Upper Eocene, Mesohippus 

 of the Lower Oligocene, Pliohippus of the Upper Pliocene, and finally 



Equus: Qua- 

 ternary and 

 Recent. 



Pliohippus : 

 Pliocene. 



Protohippus : 

 Lower Plio- 

 cene. 



Miohippus : 

 Miocene. 



Mesohippus : 

 Lower Mio- /i/^ 

 cene. 



Orohippus : 

 Eocene. 



FIG. 2. Feet and teeth in fossil pedigree of the horse. (After Marsh.) 

 a, Bones of the fore foot; b, bones of the hind foot; c, radius and ulna; d, fibula 

 and tibia; e, roots of a tooth; / and g, crowns of upper and lower teeth. 



Equus of the Quaternary and Recent. Other genera might be men- 

 tioned, but the history of this series has been pictured in a classic 



