EVOLUTION OF THE HORSE 23 



Upper Miocene. In this stage the crowns of the upper molars 

 have become much longer, the two pairs of crescents on the upper 

 molars are complete, with two half-separated cusps within the 

 inner pair. And the valleys between the crests have become 

 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 very efficient instrument for grinding hard grasses. In 

 Protohippus and Pliohippus, especially in the former, the crowns 

 of the teeth are by no means as long as in the modern horses; 

 they must therefore wear more slowly or wear out at an earlier 

 age. 



The feet in these two genera have but one toe touching the 

 ground. The side toes (second and fourth digits) are complete, 

 but much more slender than in the earlier stages and are ap- 

 parently useless, as they cannot reach the ground. In some 

 species of Pliohippus they have almost disappeared. The fore- 

 foot of Protohippus still retains tiny nodules of bone at the back 

 of the "wrist" (sometimes improperly called in the Horse the 

 " knee-joint"), which are the remains of the first and fifth digits. 



Hipparion. Pliocene. This genus, probably also a side 

 branch of the genealogical tree of the horse family, is much like 

 Protohippus, but larger and with more complication about the 

 tooth pattern. It is common in the European Pliocene beds and 

 has been found in America also. The feet are still three-toed, 

 the side toes as large as those of the older Protohippus. 



12. Equus. Pleistocene and Recent. In this stage, 

 that of the modern Horse, the side toes have entirely disappeared 

 and are represented by splints on the fore- and hind-foot. No 

 trace remains on the forefoot of the little nodules which in Proto- 

 hippus represented the first and fifth digits. The crowns of the 

 teeth are much longer than in the last stage, and of the two half- 

 separated inner columns on the upper molars, one has disap- 

 peared, the other has increased in size and changed in form. 

 The skull has lengthened and the animal is much larger. 



