1042 



THE UNGULATES, OR HOOFED MAMMALS 



Although in the members of the present group the number of toes in the foot 

 is frequently three, it may be increased to four or diminished to one; yet in all these 

 variations the symmetry of the third digit is preserved. And it is on account of the 

 prominence of this same digit that the group has received the designation of the 

 Odd-Toed, or Perissodactyle Ungulates. 



Another distinctive feature of this group is to be found in the conformation of 

 the astragalus of the ankle joint of the hind-foot. This 

 bone, which forms the upper right-hand corner of the 

 accompanying figure of the hind-foot of a rhinoceros, is 

 characterized by its deeply-grooved pulley-like superior 

 surface, while inferiorly it is abruptly truncated; and, 

 unlike that of the Even-Toed group, it has not a facet for 

 articulation with the fibula, or smaller bone of the leg. 

 The astragalus of an Even-Toed Ungulate is, on the other 

 hand, a more elongated bone, with its lower surface 

 highly convex, and divided into two distinct moieties. 

 A third very important characteristic of the limbs of the 

 Odd-Toed Ungulates is that the femur, or bone of the 

 upper segment of the hind-leg, is furnished with a pro- 

 tecting crest on the upper part of its hinder surface 

 known as the third trochanter; this trochanter (of which 

 the position is clearly shown in the left hind-limb of 

 the figure of the skeleton of the tapir) being quite un- 

 BONES OF THE RIGHT HIND- known among the Even-Toed Ungulates. 

 FOOT OF AN EXTINCT The foregoing characteristics of the feet are alone 



sufficient to distinguish the Odd-Toed Ungulates from the 

 Even-Toed group, but there are also certain other features 

 especially some connected with the teeth which it is advisable to notice. As 

 regards the cheek-teeth, it may be observed that in the upper jaw the premolars (as 

 shown in the accompanying figure) are generally as complex as the molars, whereas 

 in most members of the Even- 

 Toed group they are simpler. 

 Then, again, all the upper 

 cheek-teeth, with the exception 

 of the first, in most of the 

 earlier and more primitive rep- 

 resentatives of the group are 

 characterized by carrying six 

 columns or cusps on their crowns, of which the two innermost pairs tend to unite 

 more or less completely, and thus form a pair of oblique transverse ridges, extend- 

 ing across the crown to the two outer columns; the two latter also uniting to form a 

 longitudinal outer wall to the tooth. From this primitive type of tooth all the more 

 specialized developments may be derived, and, as we shall have occasion to notice 

 later on, while the earlier forms have low-crowned molar teeth, like those represented 

 in the figure, some of the later types have the crowns greatly elongated in the 



AN 

 RHINOCEROS. 



(From Osborn.) 



THE LEFT UPPER CHEEK-TEETH OF THE ANCHITHERE. 



(From Osborn.) 



