xii PHYLUM CHORDATA 537 



whole are usually specially modified to act as organs of 

 swift locomotion over the surface of the ground, their move- 

 ments being restricted by the nature of the articulations to 

 antero-posterior movements of flexion and extension. The 

 metacarpal and metatarsal regions are relatively very long. 

 In the sub-order Artiodactyla (or cattle, sheep, antelopes, 

 giraffes, deer, camels, pigs, and hippopotami) the third 

 and fourth digits of each foot form a symmetrical pair, and 

 in the majority are the only digits that are completely devel- 

 oped. Characteristic of the ruminants are the cephalic 

 appendages known as horns and antlers. The horns of the 

 oxen, sheep, goats, and antelopes, sometimes developed in 

 both sexes, sometimes only in the males, are horny sheaths 

 supported on bony cores, which are outgrowths of the frontal 

 bones. In the giraffes the horns, which are short and occur 

 in both sexes, are bony structures covered with soft skin, and 

 not at first attached by bony union to the skull, though sub- 

 sequently becoming firmly fixed. The antlers of the deer, 

 which, except in the case of the reindeer, are restricted to 

 the male sex, are bony growths enclosed only while immature 

 in a layer of skin, the " velvet,'' covered with very soft short 

 fur. Antlers are shed annually, and renewed by the growth 

 of fresh vascular bony tissue from the summit of a pair of 

 short processes of the frontal bones, the pedicles. 



In the pigs the legs are relatively short, and the two lat- 

 eral toes of both manus and pes are fully developed, though 

 scarcely reaching the ground. The surface is covered with 

 a scanty coat of coarse bristles. There is a truncate mobile 

 snout, the anterior end of which is disc-shaped and free from 

 hairs. A remarkable feature of the males is the development 

 of the canine teeth of both jaws into large, upwardly curved 

 tusks. 



In the hippopotami the body is of great bulk, the limbs 



