DIVISIONS OF EEPTILIA. 267 



qf the fang, and is connected with the duct of the "poison-gland." 

 This is a gland (fig. 192, a), situated under and behind the eye, 

 secreting the poisonous fluid which renders the bites of these snakes 



Fig. 192. — The head of the Rattlesnake, di.ssected to .*ow the poison-gland (a) 

 and poison-fangs (/). (After Duvernoy.) 



dangerous or fatal. When the serpent strikes at any animal, the 

 poison is forced through the poison-fang into the wound, partly by 

 the contractions of the muscular walls of the gland, and partly by 

 the compressive action of the muscles of the jaws and cheeks. 



The Ophidia are usually classified in accordance with the characters of their 

 dental apparatus, aiid may be divided as follows, some minor groups being 

 omitted : (!.} The Viperina comprise the most typical of the venomous Snakes 

 ( Venenosa), and include the common Vipers { Viperidcn) and the Rattlesnakes 

 {Crotalidce), the former being confined to the Old World, whilst the latter are 

 mainly American. The common Viper (Pelias berus) occurs abundantly in 

 Europe and Northern Asia, and is capable of inflicting a severe and even dan- 

 gerous bite. The rest of the true Vipers are African and Asiatic, well-kno^vn 

 examples being the Puff-adder oi the Cape of Good Hope (Clotho), the Horned 

 Viper {Cerastes) of North Africa, and the " Tic-polonga " (Daboia RusaelUi) of 

 India. 



The true Rattlesnakes (Crotahcs) are exclusively natives of America, and 

 they are highly poisonous. The extremity of the tail in the common Rattle- 

 snake {Crotalus durissus) is furnished with a series of horny epidermic cells 

 of an undulated pyramidal shape, articulated one within the other, constituting 

 an appendage which is known as the "rattle." Before striking its prey, the 

 Rattlesnake throws itself into a coil, and shakes its rattle, as it does also when 

 alarmed. A nearly allied form (C. horridus) is found in South America. Other 

 American examples of the Croialidce are the Fer-de-lance {Trigonocephalies) 

 of the West Indies, the Copper-head {Ancistrodon contoririx), and the Water- 

 mocassin {A . piscivorus), in which there is no rattle. In India and Southern 

 Asia there are also various Snakes belonging to the Croialidce. 



Taken as a whole, the Viperine Snakes are distinguished by having only a 

 single poison-fang on each maxilla ; while this bone is short and movable, and 

 carries no other teeth. There are, however, one or more reserve-fangs behind 



