512 CHORDATA. 
for long-continued and powerful masticatory movements. 
In the same way the molar teeth in both are ridged or 
tuberculated, the ridges being worn down very early in life, 
exposing the dentine. The parts between the enamel crests 
are filled up with cement. The enamel being harder than 
either dentine or cement, it always forms rough ridges with 
complex outline, on the inner side of which rests the 
dentine, on the outer the cement. In both the horse and 
the ox the crowns of the molar teeth are much elongated, 
forming the type called hypsodont. This condition, like the 
bony orbit, has been developed within the two sub-orders, 
many of the less specialised members of each order having 
short crowned or érachydont molars. 
In this way the row of molars forms a crushing mill 
which is capable of reducing to a pulp the most siliceous 
of grasses, and the size of which largely accounts for the 
prominent facial region. The molar series is separated by 
a more or less prominent space or diastema from the front 
teeth, indicating a separation in function between the two 
series. The condyle of the mandible is transversely cylin- 
drical, and allows of some lateral but little backward 
motion, owing to the presence of a postglenoid process 
of the squamosal. 
‘Apart from these general resemblances, the differences 
are sufficiently striking. Firstly, we notice that the skull of 
the ox bears a pair of large bony processes or cores upon 
the frontal bones, which form the basis of support for the 
long hollow horns in which they are encased in the living 
animal. These horns, assisted usually by the speed of the 
animal, form the organs of defence, or even offence, of the 
large family to which the ox belongs (Bovide), whilst frontal 
organs of one kind or another (antlers, &c.) are largely 
found in the Artiodactyla ; there is no trace of them in the 
horse, which trusts to its speed, or on occasion to its kick- 
ing powers, for defence. 
Less conspicuous distinctions in the skulls are the much 
larger nasals and the presence of an alisphenoid canal 
(through which runs the main branch of the external carotid 
artery) in the horse. These two features are small and 
may appear unimportant, but they serve to distinguish the 
two large sub-orders of the Artiodactyla and Perissodactyla. 
