XXIII] CHELONIA 591 



kinds of poison clearly indicating that the poison producing power 

 has been independently evolved in different families of the Ophidia. 

 In the first, the ELAPIDAE, the maxilla is a long bone and bears in 

 front two large teeth which are grooved, to allow the secretion of 

 glands in the lip to trickle down into the wound which they make. 

 The poison acts directly on the nervous system of the victim para- 

 lysing the nerve centres which control respiration and heart action. 

 The teeth behind are not grooved. The American Harlequin 

 Snake, Elaps fulvius, belongs to this family. This Snake receives 

 its name from its brilliant colours ; it has seventeen crimson rings 

 bordered with yellow. Another family is that of the VIPERIDAE. 

 The maxilla is much shortened and bears one enormous fang, 

 which when the mouth is closed lies against the roof of the mouth : 

 when the mouth is opened the maxilla is rotated by means of the 

 ectopterygoid, so as to erect the tooth. The poison in this family 

 causes the blood of the victim to clot in its veins. The typical 

 Rattlesnake Crotalus korridus or C. atrox derives its name from 

 an appendage of about 8 to 9 loosely connected horny rings which 

 it bears at the end of its tail, the shaking of which makes a noise 

 like a rattle. This is one of the most deadly snakes known : it is 

 found all over the United States in mountainous places and enters 

 Canada. Like all CROTALINAE or Pit-vipers it has a sensory pit 

 between eye and nose. The English Adder, Vipera berus, is, like 

 all the Old World VIPERINAE, devoid of such pits. Speaking 

 generally poisonous snakes may be recognised by their extremely 

 swollen cheeks behind which there is a slightly narrowed neck, 

 which passes into the trunk. The swelling of the cheek is due to 

 the great enlargement of the poison-gland, which is a modified salivary 

 gland (see p. 655) opening into the mouth. 



Order III. Chelonia. 



The Chelonia or Turtles are the most peculiar order of the 

 Reptilia. In some respects they are nearest to the Amphibia, but 

 they are highly specialised. Their leading peculiarity is the pos- 

 session of two great shields, a dorsal the carapace and a ventral 

 the plastron, composed of bones firmly connected together, so that 

 most of the organs of the body are enclosed in a box. The horny 

 scales which cover in this box are very large and form what is 

 known as tortoise-shell. The carapace is formed of a central row of 

 neural plates which are expansions of the spines of the dorsal 

 vertebrae with a nuchal plate in front of these and a 

 behind, the two last-named being of dermal origin* 



