302 HISTORICAL PALAZONTOLOGY. 
developed that they now reach the ground. The jrs¢ digit 
(thumb or great toe) is still wanting ; as also is the 77/¢h digit 
Fig. 230.—Skeleton of the foot in various forms belonging to the family of the Eguzde. 
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 
(Zguus), 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 Ovohippus. 
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 ¢/z7¢ toe is still the largest. 
The frst toe (thumb or great toe) is still wanting ; but in this 
ancient representative of the Horses, the 7/¢A 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 guide, in which 
the first digit is developed, and the foot assumes its typical 
five-fingered condition: 
Passing on to the Even-toed or Avtiodactyle Ungulates, no 
representative of the AZzppotamus seems yet to have existed, 
but there are several forms (Cheropotamus, Hyopotamus, &c.) 
more or less closely allied to the Pigs (Swzda); and the 
singular group of the Avxoplotheride 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 
