312 



HISTORICAL PALEONTOLOGY. 



each foot, this being terminated by a single broad hoof, and 

 representing the middle toe the third of the typical five- 

 fingered or five-toed limb of Quadrupeds in general. In 

 addition, however, to this fully-developed toe, each foot in 

 the horse carries two rudimentary toes which are concealed 

 beneath the skin, and are known as the " splint-bones. " 

 These are respectively the second and fourth toes, in an 

 aborted condition ; and the first and fifth toes are wholly 

 wanting. In Plipparion (fig. 230, C), the foot is essentially 

 like that of the modern Horses, except that the second and 

 fourth toes no longer are mere " splint-bones, " hidden be- 

 neath the skin; but have now little hoofs, and hang freely, 

 but uselessly, by the side of the great middle toe, not being 

 sufficiently developed to reach the ground. In Anchitherium, 

 again (fig. 230, B), the foot is three-toed, like that of Hipparion; 

 but the two lateral toes (the second and fourth) are so far 

 developed that they now reach the ground. The first digit 

 (thumb or great toe) is still wanting; as also is the fifth digit 



.. 4 



Fig. 230. Skeleton of the foot In various forms belonging to the family of the Equidce, 

 A, Foot of Orohippus, Eocene ; B, Foot of Anchitherium, Upper Eocene and Lower 

 Miocene ; C, Foot of Hipparion, Upper Miocene and Pliocene ; D, Foot of| Horse 

 (Equus), Pliocene and Recent. The figures indicate the numbers of the digits in the 

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



(little finger or little toe). Lastly, the Eocene rocks have 

 yielded in North America the remains of a small Equine 

 quadruped, to which Marsh has given the name of Orohippus. 

 In this singular form which was not larger than a fox the 

 foot (fig. 230, A) carries four toes, all of which are hoofed and 

 touch the ground, but of which the third toe is still the largest. 



