HIPPOPOTAMUSES, PIGS, AND DEER 283 



side, as in the camels, or disappear absolutely, as is the case 

 with the tragulines and Pecora. Canine teeth tend to disappear 

 in the upper jaw, though they are retained by the camels and 

 tragulines, and by some deer. In the lower jaw a marked 

 feature of this section of the Artiodactyles is that the canine 

 tooth grows close up to the lower incisors, and tends to resemble 

 them exactly in appearance. In the lower jaw of an ox, for 

 instance, the unlearned observer would decide that there were no 

 canine teeth at all, but four incisors on each side instead of the 

 normal three. As a matter of fact, the outer tooth in this range 

 is a canine. The molars also develop an ever-increasing length 

 of crown ; many of them are hypsodont, as in the horse. More- 

 over, all the molar teeth of camels and Pecora are what is called 

 " selenodont," that is to say, their enamel ridges assume a crescent 

 shape, so that the grinding surface is a series of half-moon-shaped 

 ridges, instead of a number of blunt hillocks as in the swine. 



The Even-toed Ungulates described below are those now 

 existing in the British Islands, or which are known to have 

 inhabited some portion of this area since man came on the 

 scene at the beginning of the Pleistocene period. 



Family: HIPPOPOTAMID^. HIPPOPOTAMUSES 



The Hippopotamuses are for the most part^ huge animals, 

 leading an amphibious life. The tusks, or canine teeth, are 

 enormously developed, especially in the lower jaw. The incisor 

 teeth are also prolonged into tusks. The extremity of both jaws, 

 especially that of the lower, is remarkably broad, and in the lower 

 jaw the canines and incisors as seen from the front are in the same 

 line, that is to say, the canines are pushed so far forward in their 

 growth that they are in no way behind the incisors, as is the case 

 in a normal mammalian arrangement of the teeth. The feet are 

 four-toed, the two central toes being larger and more important 

 than the second and fifth. The skin is bare of hair, with the 



1 Some of the forms isolated in Cyprus and Malta during the Pleistocene 

 degenerated into pygmy types hardly larger than small pigs. 



