THE EOCENE PERIOD. 313 



The 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 Equidcc, in which 



Fig. 231. A noplotfierium commune. Eocene Tertiary, France. ( After Cuvier.) 



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 Hippopotamus 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 Anoplotherida 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 

 peculiarity that they are arranged in a continuous series, with- 

 out any gap or interval between the molars and the canines ; and 

 the back teeth, like those of all the Ungulates, are adapted for 

 grinding vegetable food, their crowns resembling in form those 

 of the true Ruminants. The genera Dichobune and Xiphodon, 

 of the Middle and Upper Eocene, are closely related to 

 Anoplotherium, but are more slender and deer-like in form. 

 No example of the great Ruminant group of the Ungulate 

 Quadrupeds has as yet been detected in deposits of Eocene 

 age. 



