xii REPTILIA 421 



whole limb being reduced to a tiny conical vestige. In still other li/. 

 such as the " Blind-worm " or " Slow-worm " (Anguis) and Amphis- 

 haena, a lizard which has taken to burrowing like an earthworm, the 

 limbs have disappeared entirely and the whole appearance has become 

 snake-like or worm-like. 



The skin of the reptile provides one of its most conspicuous charac- 

 teristics. The dry horny epidermis, practically devoid of glands, is 

 given flexibility by being subdivided up into scales the highest degree 

 of flexibility being attained by the scales overlapping like those of a 

 teleostean fish (Lizards, Snakes). But it must be borne in mind that 

 the scale of the reptile is in its nature essentially different from that of 

 a fish the former being a plate of cornified epidermis with a backing 

 of tough dermis, while the latter is a plate of bone, the epidermis covering 

 it being thin, soft and glandular. Horny claws are present on the digits. 



The horny covering layer of the skin is shed from time to time. In 

 the Rattlesnakes the portion ensheathing the tip of the tail remains for 

 a time loosely attached to the body, a chain of these loose pieces con- 

 stituting the " rattle." Various other poisonous snakes make their 

 presence known by causing the tip of the tail to vibrate rapidly amongst 

 dry grass or twigs, and the special arrangement of the Rattlesnake marks 

 a further stage of evolution of this habit. 



In connexion with the alimentary canal the first thing to be noticed 

 is that in all terrestrial vertebrates there are special glands opening into 

 the buccal cavity and serving by their fluid secretion to keep the mouth 

 lining moist. In some of the reptiles the secretion has become highly 

 poisonous. In one of the Lizards (Heloderma of Mexico, New Mexico, 

 and Arizona) this applies to a series of glands opening near the bases of 

 the grooved teeth of the lower jaw. It is, however, amongst the snakes, 

 particularly the Vipers (including the Puff Adder of Africa, the Adder 

 of Europe and Asia, the Moccasin-Snake or Copper-head of the United 

 States, the Fer-de-lance and Vivora-de-la-Cruz of South America, and 

 the Rattlesnakes of North and South America) that the poison-apparatus 

 reaches its highest development. 



In these snakes there is on each side a functional poison-fang. Each 

 of these is a much elongated, sharply pointed tooth with a deep groove 

 along its anterior face. In the true Vipers this groove is closed in to 

 form a canal except at its two ends, close to the base and close to the tip 

 of the tooth respectively. The fang is mounted upon the maxillary bone 

 and this, which bears no other functional teeth, is compact in form, 

 being greatly shortened in an antero-posterior direction. It is suspended 

 from the skull by a hinge-joint at its dorsal end, about which it can swing 



