VERTEBRATA : REPTILIA. 54! 



they are anchylosed), and the poison is forced through the 

 tube which perforates each, partly by the contractions of the 

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



Fig 298. The head of the Rattlesnake, dissected to show the poison-gland (a) and 

 poison-fangs (f). (After Duvernoy.) 



jaws. In most poisonous Snakes the superior maxillae carry 

 no other teeth except the poison-fangs and their rudimentary 

 successors, but in some cases there are a few teeth behind the 

 fangs ; whilst the palatine teeth are always present, as in the 

 harmless species. In some other venomous Snakes, again 

 (e.g., Naja and the ffydrophidx), the jaws and teeth agree in 

 most characters with those of the non-venomous Snakes, but 

 the first maxillary teeth are larger than the others, and form 

 canaliculated fangs. Lastly, in a few forms the terminal maxil- 

 lary teeth are deeply canaliculated, but are not connected with 

 the duct of any poison-gland. 



Fourthly, in all the Serpents the teeth are anchylosed 

 with the jaw, and are never sunk into distinct sockets or 

 alveoli. 



The Ophidia are usually classified in accordance with the 

 characters of their dental apparatus, and may be divided as 

 follows, some minor groups being omitted: (i.) The Viperina 

 comprise the most typical of the venomous Snakes ( Venenosd), 

 and include the common Vipers ( Viperidat) and the Rattle- 

 snakes (Crotalidce), the former being mostly confined to the 

 Old World, whilst the latter are mainly American. The com- 

 mon Viper (Pelias berus] occurs abundantly in England and 

 Scotland, and is capable of inflicting a severe and even danger- 

 ous bite, though it does not appear that fatal effects commonly 

 follow except in the case of children or subjects previously 

 debilitated. The true Rattlesnakes (Crotalus) are exclusively 

 natives of America, and they are highly poisonous. The extre- 



