HISTORY OF THE PERISSODACTYLA 



305 



Fig. 154. — Scries of horse skulls in ascending geological order. A., iEohippus, lower 

 Eocene (after Cope). B., i Mesohippus, lower and middle Oligocene. C, i Proto- 

 hivpus, upper Miocene (after Cope). D., Equus. 

 X 



