702 



MANUAL OF ZOOLOGY. 



remains of fossil Equidce. Many of these are of special in- 

 terest, as showing an almost perfect series of gradations be- 

 tween a foot with three complete toes and a foot with only one 

 complete digit. Some of them also exhibit other curious tran- 

 sitional characters. 



The most ancient type of the Equid<z is the Eohippus of the Lower 

 Eocene of North America, in which the fore-feet have four complete toes 

 and a rudimentary pollex, while the hind-feet have three toes. 



Orohippus is the next oldest known Equine genus, and comprises small 

 mammals about as big as foxes, with the fore-feet four- toed (fig. 397, A), 



B 



4 



Fig. 397. Skeleton of the foot in various forms belonging to the family of the Equidce: 

 A, Foot of OrohippriS) Eocene ; B, Foot of A nchithertum, Upper Eocene and 

 Lower Miocene ; C, Foot of Hipparion, Upper Miocene and Pliocene ; D, Foot of 

 Horse (Eguus), Pliocene and Recent. The numerals indicate the numbers of the 

 digits in the typical five-fingered hand of Mammals. (After Marsh.) 



and the hind-feet three- toed. In the fore-foot, the pollex alone is wanting, 

 but the middle toe is much the largest. The genus is from the Eocene of 

 North America. 



In the Miocene Tertiary occur the genera Anchitherium, Miohippus, 

 and Mesohippus, all of which have three toes to both feet. Mesohippus 

 has an additional "splint-bone" (rudimentary metacarpal, or metatarsal) 

 representing a fourth toe. Miohippus, about as big as a sheep, has the 

 three toes sub-equal, and all touching the ground. Anchitherium (fig. 

 397, B) has the middle toe much the largest, though the lateral toes still 

 reach the ground. 



In the later Miocene and earlier Pliocene we find the genus Hipparion, 

 in which the foot is still three-toed (fig. 397, C) ; but the middle toe is 

 alone functionally useful, the two lateral toes, though appearing externally, 

 not being long enough to touch the ground. 



In the later Pliocene we meet with the genus Pliohippus, in which the 

 foot is precisely that of Equus, with the lateral toes reduced to splint-bones 

 (fig. 397, D), but there is an additional prsemolar, and an "antorbital fossa" 

 is present. Lastly, in the Post-pliocene appears the genus Eqmts itself. 



ARTIODACTYLE UNGULATES. 

 SECTION B. ARTIQ.DACTYLA. In this section of the Ungu- 



