512 



THE HORSE'S POSITION IN THE ANIMAL WORLD 



third, fourth, and fifth digits. It will be observed that the third or 

 middle digit is the largest of the four, representing in fact what has 

 previously been termed the one big digit of the horse. 



The Protorohippus has a well-developed ulna, a well-developed fibula, 

 and short-crowned grinders of simple pattern. 



Comparing its skeleton (Plate LXX) with that of the horse, we see 

 that there is a general correspondence in grace and delicacy of outline in 

 the two animals. 



The next drawings (fig. 3, Plate LXXI) represent the fore- and 

 hind-feet of the Mesohippus, from the Lower Miocene immediately suc- 

 ceeding the Eocene in which the Protoroliippus was found. In comparing 

 this set of figures with those last described, it will be seen that only three 

 prominent digits remain in both the fore- and hind-feet, the fifth dioit 



Fig. 669. — Radius (r) and Ulna (u) of Fossil Ancestors of the Horse, showing the gradual 

 diminution in relative size of the ulna (not to scale). 1, Phenocodus; 2, Protorohippus; 

 3, Mesohippus; 4, Miohippus (Anchitherium); 5, Protohippus; 6, Horse. 



being reduced to a very .slender rudiment. In this animal the ulna 

 (fig. 669) is well developed, but the fibula has become quite rudimentary; 

 the forms of the molar teeth have not undergone very considerable 

 chano-e. 



In the Miohijjpus, the contemporary of the Anchitherium of Europe, 

 the extremities remain nearly as in the Mesohippus. The ulna has the 

 lower part greatly reduced; the other limb -bones remain nearly the 

 same. 



To the Miocene period also belongs the Merychippus, found throughout 

 a large portion of North America, which is remarkable in that in the young 

 condition it has the short- crowned, uncemented teeth of its ancestors, 

 while the adult animal has the long-crowned and cemented teeth of its 

 successors. 



Proceeding upwards to the Protohippus and its near relative the 

 European Hipparion from the Lower Pliocene, it appears that the changes 

 cliiefiy relate to the ulna, which in these animals has decreased consider- 

 ably in length, only reaching to the middle of the radius. The two 



