4 THE HORSE, ASS, AND MULE 



8. Anchitherium. From Lower Miocene period. Is much Hke 

 Mesohippus, but is larger and has the crests of the teeth some- 

 what higher and more complete. This form may not be in direct 

 line of descent, but may be a side branch. Found in both Europe 

 and America. 



9. Parahippus and Hypohippus. From Middle Miocene period. 

 The tooth crests in Parahippus are much higher, the upper molars 

 are changing in form, and a second pair of crescents is formed. 

 Hypohippus is off the direct line of descent. A complete skeleton 

 of this form, now in the American Museum, was discovered in 1901 

 in Colorado. This species attained the size of a Shetland pony. 

 The fore foot has small rudiments of the first and fifth toes, but 

 there is no splint of the fifth, as in Mesohippus. The second and 

 fourth toes lightly touch the ground. It has been called the "forest 

 horse," and is supposed to have lived on lowlands and in forests. 



10. Protohippus. From Middle and Upper Miocene period. 



11. Pliohippus. From Middle and Upper Miocene period. 



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. 

 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. Thus the tooth 

 surface has a series of enamel ridges always projecting somewhat 

 above the grinding surface, because the softer material on each 

 side wears down into hollows, yet never breaks off. This makes 

 an efficient medium for grinding grass. In these two forms of 

 horse, notably Protohippus, the crowns of the teeth are not as long 

 as in the modern horse. The feet in these two have but one toe 

 touching the ground. The side toes are complete, but are more 

 slender than in the earlier stages and are apparently useless. In 

 some species of Pliohippus they have almost disappeared. Horses 

 at this period wrere about forty inches, or ten hands, high. 



Hipparion. From Pliocene period. This is similar to Proto- 

 hippus, but with larger and more complicated tooth patterns. In 

 this period some horses have three toes and others one. 



12. Equus. From Pleistocene period and Recent. This is the 

 stage of the modern horse. The side toes are gone but are 



