Plate X. 



a. Recent. 



Equiu. 



b. Pliocene. 



1. Pliohippus, 



2. Protohippus. 



(Hipparion. 



c. Miocene. 



1. Miohippus. 



(Anchitherinm.) 



2 Mesohippus. 



d Eocene. 



Orohippus. 



Illustrations of the characters of successive Genera under 

 the Horse type. (From Marsh.) 



