ODD-TOED GROUP. 



455 



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 

 characterised 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- 

 jecting 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- 



known among the Even-toed Ungulates. 



BONES OF THE BIGHT HIND-FOOT 

 OF AN EXTINCT RHINOCEROS. 

 (From Osborn.) 



THE LEFT UPPER CHEEK-TEETH OF THE ANCHITHERE. (From Osborn.) 



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 accom- 

 panying 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 representatives of the group are characterised 

 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, extending 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 specialised 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 vertical direction. In this respect, therefore, the Odd-toed 

 Ungulates have developed in a manner exactly paralleled among the Even-toed 

 group, a similar parallelism being also noticeable in respect to the reduction of 

 the number of toes on the feet. Moreover, as we find in the Even-toed Ungulates 



