MAMMALIA. 571 
SUB-ORDER IV.—PERISSODACTYLA. 
A good deal has already been said concerning the 
FPerissodactyla in the chapter upon the Horse and Ox, in 
which this sub-order is contrasted with that of the Avzo- 
dactyla (page 509). 
The main structural features of the sub-order are as 
follows :— (1) The molar teeth are bilophodont, or with 
complex crowns derived from the bilophodont condition. 
(2) Dorso-lumbar vertebrz, usually twenty-three in number. 
(3) The femur has a third trochanter. (4) In the skull the 
nasals are large and there is an alisphenoid canal. (5) The 
carpus and tarsus are alternate and the toes are never more 
than four, mostly three or one, but in all cases the main 
axis of support passes through tibia, astragalus, navicular, 
and third toe. (6) Stomach simple. (7) Diffuse placenta 
and mammee inguinal. 
The molar teeth pass, in the group, from the simple 
brachydont bilophodont condition (derived, as shown, page 
462, from the tubercular type) to the complex hypsodont 
type with cement added. 
The third trochanter is preserved in this group from the 
early condylarthrous ancestors, and the disappearance of 
the toes can be traced upwards within the group. No 
modern Perissodactyla have five toes, but the tapir has four 
in the fore-foot, the pollex being lost, the rhinoceros has 
three and the horse merely the one. The main axis passing 
through tibia, astragalus, navicular and third toe, it naturally 
follows that the fibula is reduced or at least loses its articu- 
lation with the calcaneum, and the astragalus has nearly all 
its distal articular surface attached to the navicular. In the 
front-limb the os magnum becomes more and more pro- 
minent as the third toe usurps the functions of the others. 
In the simplicity of the stomach and the diffuse placenta 
the Perissodactyla appear to present more primitive char- 
acters than the Artiodactyla. (As has been noticed, there 
has been a great deal of parallel evolution in these two sub- 
orders. The common characters thus acquired form a basis 
for the institution of the group Ungulata Vera containing 
these two sub-orders, in contrast with the three preceding 
sub-orders as Sub-Ungulata. Such a classification, based 
upon parallel evolution, must, however, be unnatural.) 
