THE TEETH OF REPTILES. 255 



keep teeth of the one series from getting over to the other 

 side, and probably serves to hold in place the reserve tooth 

 when the older tooth is erected for biting. 



The reserve poison fangs, as many as ten in number in 

 the Rattlesnake, are likewise arranged in two parallel series, 

 in which the teeth exist in pairs of almost equal age ; the 

 tooth in use is thus derived alternately from the one and 

 the other series, as is indicated by the consecutive numbers 

 in the figure, a septum of connective tissue keeping the two 

 series of teeth distinct from one another. 



The teeth being arranged in pairs of almost equal age, 

 suggest that the succession is both rapid and regular. All 

 the reserve teeth lie recumbent in and behind the sheath 

 of mucous membrane which covers in the functional tooth. 



This arrangement of the successional teeth in a paired 

 series does not exist in the Cobra, in which the successional 

 teeth form but a single series ; perhaps this may serve to 

 explain the preference of the snake charmers for the Cobra, 

 which would probably take longer to replace a removed 

 poison fang than a viperine snake would. 



But in the colubrine venomous snakes the successional 

 poison fang sometimes makes its way to a spot a little to the 

 side of its predecessor, so that there may possibly be no loss 

 of time : and notwithstanding that they are in a measure 

 transitional forms between the harmless and the viperine 

 snakes, some of them are most virulently poisonous and 

 deadly in their bite Q. 



This arrangement of tivo distinct chains of younger de- 

 veloping organs, all destined to keep the creature always 

 supplied with one organ in a state of efficiency, is, so far as 

 I know, without parallel. 



Like other ophidian teeth the poison fangs become an- 



(*) I have given a more detailed account of the succession of poison fangs 

 in the Philos. Trans., 1876, Part i. 



