4 THE HORSE, A$S, AND MULE 



about the size of a small dog or fox. The complete skeleton of 

 a Protorohippus was found in 1880 in Wyoming and is now in 

 the American Museum collection. 



Epihippus is from the Upper Eocene period. Only incomplete 

 specimens have been found. There are four toes in front and 

 three behind, but the central toe in each case is larger, stronger, 

 and more important than the side toes. The teeth have gone 

 through changes, with more crescents and crests. 



Mesohippus is from the Oligocene period. There are three toes 

 on each foot and a splint representing the fourth toe of the fore foot. 

 The middle toe is now much larger than it was in the preceding 

 stage,, the side toes bearing but little weight of the body. There 

 is an increased development of the molars. In the Middle Oligo- 

 cene is found one species about the size of a coyote, or eighteen 

 inches high, while in the Upper Oligocene is another species as 

 large as a sheep. Several complete skeletons have been found. 



Protohippus is from the Miocene period. A number of different 

 species existed in the Lower, Middle, and Upper Miocene periods. 

 The side toes are slender and no longer reach the ground. In 

 some species they have completely disappeared. The teeth are 

 more like those of the modern horse the crowns being much 

 lengthened, and the crests and ridges higher and more com- 

 plicated, with cement between the ridges. Horses of this period 

 ranged in size from a mastiff dog to a Shetland pony. 



Pliohippus is from the Pliocene period. Comparatively little is 

 known of this form, excepting that it was either one-toed or 

 very nearly so. 



Equus is from the Pleistocene period and Recent. The side toes 

 are gone, but are represented by splints on fore and hind foot. 

 On the fore foot no trace of the first and fifth toes, found on the 

 Protohippus, remains. The crowns of the teeth are much longer, 

 and the teeth have changed otherwise. The skull has lengthened, 

 and the horse is much larger. 



Tracing the evolution of the horse, we find that this animal 

 gradually increases in height, the number of toes reduces to one 

 on each foot, while the teeth increase in length and crown de- 

 velopment. With increase in height comes a gain in the propor- 

 tionate length of the limbs, more especially in the lower part, 



