454 



AN INTRODUCTION TO ZOOLOGY 



in the directions ^ of becoming forest dwellers. On the other 

 hand, Merychippus from the middle and upper layers, while it 

 also shows a slight increase in size, is more important in that it is 

 modified for a life on the plains. Its permanent teeth are adapted 

 lor a grass diet, while its milk teeth still retain the characteristics 



FIG. 161. Restoration of the Miocene prairie 

 horse, Merychippus. 'After Lull. 



^. 



of an animal feeding on shrubs and softer 

 vegetation. Further, while still three toed, 

 in ordinary locomotion its lateral digits 

 did not reach the ground. In the upper 

 Miocene appears Protohippus, which stood 

 36 inches at the shoulder. There is not 

 much structural advance shown save that 

 the lateral digits are slightly smaller and 

 further removed from the ground. The 

 principal point of interest is the teeth, 

 whose enamel has become more complex, 

 and even the milk teeth are of the grass- 

 eating type. 



Pliocene. This period is represented 

 by Pliohippus, which first occurs in the middle beds. 

 48 inches at the shoulder, a fairly large modern horse being 60 inches, 

 i.e. 14 hands, and was more horselike in its general proportions. 

 In this animal the two lateral toes on fore and hind feet become 

 more reduced, and in some of the species have almost entirely 

 disappeared, or better become nearly as ludimentary as in our 

 modern horse. 



FIG. 162. Hand (A) and 

 foot (B) of the first one- 

 toe horse, Pliohippus 

 pernix, Pliocene, Ne- 

 braska. One-fourth 

 natural size. After 

 Lull. 



It was 



