310 OUTLINES OF EVOLUTIONARY BIOLOGY 



Hyracotherium, remains of which have been found in the 

 London Clay (Lower Eocene). Another Lower Eocene genus 

 was Eohippus, which seems to have arisen in Western Europe, 

 possibly from a hyracotherian ancestry, and migrated, by way 

 of Northern Asia, to America, where its remains occur in rocks 

 of the same age. In this animal (Fig. 154) the fore foot had 

 four well developed digits and the thumb was represented by a 

 splint bone ; in the hind foot the great toe had entirely dis- 

 appeared and the fifth digit was represented only by a splint 

 bone. In both fore and hind feet 

 the third or middle digit was already 

 conspicuously larger than any of the 

 others. 



Eohippus was succeeded by Pro- 

 torohippus (Fig. 153, a), which was 

 some 3 inches higher and had lost 

 the vestigial thumb. Then came 

 Orohippus (Fig. 153, b), again a little 

 larger and with closely similar feet 

 (Fig. 155), but with a considerable 

 advance in the evolution of the 

 grinding teeth. The last of the 

 Eocene horses was Epihippus, still 

 " li) /V with four toes in front and three 

 i behind, but with the lateral toes 



b further reduced in size and another 



FIG. 154. a, Fore Foot and distinct advance in tooth structure. 



b, Hind Foot of Eohippus i n Qligocene times there occurred 



after Marsh*) in N rth America Mesohippus and 



Miohippus, and in Europe Anchi- 



therium. Mesohippus (Fig. 153, c) was 18 inches or more in 

 height, with three digits and a vestige of the fifth in the fore 

 foot and three digits only in the hind foot (Fig. 156). Miohippus 

 attained a height of 24 inches and closely resembled Mesohippus 

 in the structure of its feet. Anchitherium is supposed to be 

 a European derivative of Miohippus. 



In the Miocene period the horses appear to have attained their 

 maximum of development as a group, and a number of extinct 

 American genera are distinguishable. Merychippus (Fig. 153, d), 

 Protohippus and Neohipparion were still three-toed horses, though 

 the lateral digits were now greatly reduced (Fig. 157). Pliohippus 



