Equus : Qua- 

 ternary and 

 Recent. 



Pliohippus : 

 Pliocene. 



Protohippus : 

 Lower Plio- 



Miohippus : 

 Miocene. 



Mesohippus : 

 Lower Mio- 

 cene. 



Orohippus : 

 Eocene. 



PLATE 47. Diagrams illustrating gradual changes in foot structure and pattern of ridges on 

 the crowns of the molar teeth in fossil and recent species of the horse family. [After MARSH.] 



a. Bones of the fore foot. b. Bones of the hind foot. c. Bones of the fore arm (radius and 

 ulna), d. Bones of the lower leg (tibia and fibula), e. side view of molar tooth, f, g. grinding 

 surfaces of upper and lower molar teeth, showing the grinding ridges. 



