h'\()Li rio.s OF rill': iiowsh: i.\ .wivta'a; 21 



lion. Tlu'sc \(My perfect specimens show a vesti<ie of the fifth di^it in 

 the lofe foot still preser\-e(l as a tiii>' nodule of hone at the hack of the 

 wrist . 



11. Hipparion, Protohippus and Pliohippus. I'im'kh Mkx i:.\k 

 AND Pliocenk. These three closely I'elated <i-enei'a i-epicsent the latest 

 stable of three-toed horses, before the side toes were reduced to splints. 

 The teeth are l()nj:;-cro\vned, both milk and i)ernianent teeth beinjj; 

 heavily cemented, and the side toes are extremely slender. They first 

 appear in the ri)per Miocene, i^robably directly descended from Middle 

 Miocene species of Mcrijchippus, and are best represented by the beauti- 

 fully preserved skeleton of ////^/^r/r/o// whittieyi U-oiu Little White River, 



Fig. 11. Menjchippus. Middle Miocene. I'pper teeth, one-half natural size. 

 Moderately long-crowned teeth, well cemented 



South Dakota, found by Mr. H. F. Wells of the Whitney expedition of 

 1902. This species, except for the very large head, had the graceful 

 and slender proportions of the antelopes, but in Protohippus and es- 

 pecially in Pliohippus the skeleton approached more nearly the stockier 

 proportions of the modern horses. The Hipparion whitneyi is regarded 

 by Professor Osborn as fitted to live in a semi-desert country, and in 

 contrast to the Hypohippus, is called the '' three-toed desert horse." 

 The restoration depicts this adaptation and environment. 



In this stage the crowns of the upper molars have l)ecome nuich 

 longer, the two pairs of crescents on the upper molars are complete, with 

 two half-separated cusps within the inner pair. And the valleys Ix^tween 

 the crests are completely filled with cement, so that with the wear of the 

 teeth the edges of hard enamel are backed inside by dentine and outside 

 by cement. In this way the surface of the tooth has a series of enamel 

 ridges always projecting a little above the grinding surface, because the 

 softer material on each side wears down into hollows, yet never breaking 

 off, because they are braced so thoroughly on each side. This is a ver}^ 

 efficient instrument for grinding hard grasses. The crowns of the teeth 

 in these Miocene horses are by no means as long as in the modern horses; 

 they must therefore have worn more slowly or worn out at an earlier 

 age. 



The feet in these genera have but one toe touching the ground. The 

 side toes (second and foiu-th digits) are complete, but much more slender 



