790 GEOLOGY 



the lateral toes fell out of use and were atrophied. The first class I 

 reached its extreme type at length in the horse, and the second in] 

 our cloven-hoofed cattle; but these perfected types were not 

 attained in the Eocene, for the evolution of the perissodactyls did 

 not pass beyond three-toed forms in the Eocene. 



The horse has become a classic example of evolution. The 

 earliest recognized form was the H yracotherium (Fig. 531), wli 

 equine characters were obscure. Pachynolophus represented a 



Fig. 530. Dinoceras mirabile, restoration of skeleton by Marsh ; about 13 

 feet long, Middle Eocene, Wyoming. 



slight advance, and the Orohippus (Epihippus) a greater one. The 

 latter was four-toed (three functional) in front and three-toed bo- 

 hind, and the limbs and teeth were slightly modified in the direction 

 of the horse. These forms were about the size of a small dor. and 

 as much canine as equine in appearance. The evolution continued 

 through the remaining periods of the Tertiary, true horses appear- 

 ing in the Pliocene. The primitive Eocene forms lived both in 

 Europe and America, and the evolution followed similar li 

 the two continents. 



The rhinoceros family appears in the record in the later part of 

 this period, but had its development chiefly in the next. 



Artiodactyls emerged from their generalized beginnings nmro 

 slowly. Suina (pigs, peccaries, hippopotamuses) were repre- 



