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. 
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 the left upper cheek-teeth of the anchithere. (From Osborn.) 
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 
rideres, 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 maybe 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 
BONES OF THE BIGHT HIND-FOOT 
OF AN EXTINCT RHINOCEROS. 
(From Osborn.) 
