512 CHORD AT A, 



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 brachydont 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 {Bovidcs), 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 alispJufioid 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 Ferissodactyla, 



