OPHIDIA. 405 



and conveying the duct of the so-called poison-gland. This 

 is a gland, probably produced by a modification of one of the 

 buccal salivary glands, situated behind and under the eye on 

 each side, and secreting the fluid which renders the bite of 

 these snakes dangerous or fatal. When the animal strikes its 

 prey, the poison-fangs are erected, and the poison is forced 

 through the tube which perforates each, partly by the con- 

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

 muscles of the jaws. In most poisonous snakes the superior 

 maxillae carry no other teeth except the poispn-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. Some of the most deadly 

 snakes, too, carry upon the upper maxillae long grooved or 

 canaliculated fangs, which cannot be raised or depressed at 

 will, and which have smaller solid teeth behind them. Others, 

 again, not certainly known to be poisonous, have canaliculated 

 fangs placed far back upon the superior maxillae, with small 

 solid teeth in front of them. , 



Fourthly, in all the Serpents the teeth are anchylosed to the 

 jaw, and are never sunk into distinct sockets or alveoli. 



A good classification of the Ophidia is still a desideratum, 

 and probably, in the meanwhile, the one proposed by Dr Gray 

 is the best. This eminent naturalist divides the snakes into 

 the two sub-orders of the Viperina and Colubrina, the former 

 having only two perforated poison -fangs on the superior 

 maxillae, whilst these bones in the latter carry solid teeth, 

 either with or without additional canaliculated fangs. 



The sub-order Viperina comprises the common Vipers 

 ( Viperida), and the Rattlesnakes ( CrotaUd<K\ the former being 

 wholly confined to the Old World, whilst the latter are mostly 

 American. The common Viper (Pelias berus) occurs abun- 

 dantly in England and Scotland, and is capable of inflicting 

 a severe and even dangerous bite, though it is doubtful if fatal 

 effects ever follow except in the case of children or subjects 

 previously debilitated. The Rattlesnakes are almost exclu- 

 sively natives of America, and they are highly poisonous. The 

 extremity of the tail in the true Rattlesnake (Crotalus horridus) 

 is furnished with a series of horny epidermic rings, constituting 

 an appendage which is known as the "rattle." This produces 

 a kind of rattling noise when the animal moves, but the exact 

 object of this appendage is a doubtful one. The head of the 

 Viperine Snakes (figs. 157, 158) is broad, somewhat triangular 

 in shape, broadest at its middle, and showing a very distinct 

 line of demarcation between the head and neck. The head 



