HISTORY OF THE PERISSODACTYLA 



305 



Fig. 1S4. — Series of horse skulls in ascending geological order. A., ^ Eohippus, lower 

 Eocene (after Cope). B., t Mesohippus, lower and middle Oligocene. C, tProto- 

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



■ X ^ 



