302 



HISTORICAL PALAEONTOLOGY. 



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 Eqitid(p. 

 A, Foot of Orohippus, Eocene ; B, Foot of Anchitherinm, 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. 

 IL\\Q first toe (thumb or great toe) is still wanting; but in this 

 ancient representative of the Horses, the fifth or " little " toe 

 appears for the first time. As all the above-mentioned forms 

 succeed one another in point of time, it may be regarded as 

 probable that we shall yet be able to point, with some cer- 

 tainty, to some still older example of the Equidcz, in which 

 the first digit is developed, and the foot assumes its typical 

 five-fingered condition. 



Passing on to the Even-toed or Artiodactyle Ungulates, no 

 representative of the Hippotamm seems yet to have existed, 

 but there are several forms ( Charopotamus^ Hyopotamus, &c.) 

 more or less closely allied to the Pigs (Suida)\ and the 

 singular group of the Anoplotheridcz may be regarded as form- 

 ing a kind of transition between the Swine and the Ruminants. 

 The Anoplotheria (fig. 231) were slender in form, the largest 

 not exceeding a donkey in size, with long tails, and having the 

 feet terminated by two hoofed toes each, sometimes with a 

 pair of small accessory hoofs as well. The teeth exhibit the 



