xii PHYLUM CHORDATA 531 



number of digits is more or less reduced, and the limbs as a 

 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 suborder Artiodactyla (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 completely developed 

 digits. 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 andoc cur 

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

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

 quently 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 im mature 

 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, \^& pedicles. 



In the Pigs the legs are relatively short, and the two lateral 

 toes of both manus and pes are fully developed, though 

 scarcely reaching the ground. The surface of the whole 

 animal is covered with a scanty coat of coarse bristles. 

 There is a truncated 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. 



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