64 INTRODUCTION 



the poison duct runs above the latter, making a bend 

 only at its anterior extremity, and the tranverse bone 

 has not the same action on the erection of the fangs. 

 Otherwise the mechanism is the same. 



In the Opisthoglyphous Colubrids, with grooved 

 teeth situated at the posterior extremity of the 

 maxilla, a small posterior portion of the upper 

 labial or salivary gland is converted into a poison- 

 secreting organ, distinguished by a light yellow 

 colour, provided with a duct larger than any of 

 those of the labial gland, and proceeding inward 

 and downward to the base of the grooved fang ; the 

 duct is not in direct connexion with the groove, but 

 the two communicate through the mediation of the 

 cavity enclosed by the folds of mucous membrane 

 surrounding the tooth, and united in front. 



The reserve or successional teeth, which are 

 always present just behind or on the side of the 

 functional fang of all venomous snakes, are in no 

 way connected with the duct until called upon to 

 replace a fang that has been lost. It could not be 

 otherwise, since the duct would require a new 

 terminal portion for each new fang; and as the 

 replacement takes place alternately from two parallel 

 series, the new poison-conveying tooth does not 

 occupy exactly the same position as its predecessor. 



Two genera, Doliophis among the Elapine Colu- 

 brids, and Causus among the Viperids, are highly 

 remarkable for having the poison gland and its duct 



