644 



OPHIDIA 



Sub-Fam. 2. Crotalinae (" Pit-Vipers "). With a deep cavity 

 or pit between the eye and the nose, lodged in the hollowed- 

 out maxillary bone. This pit is lined with a modified continua- 

 tion of the epidermis, and is amply supplied 

 with branches from the trigeminal nerve. It 

 is undoubtedly sensory, but we do not know its 

 function. A good anatomical account of this 

 organ has been given by West. 1 Some of the 

 Pit- Vipers have a rattle at the end of the tail ; 

 these are the Eattle-Snakes. The rattle is 

 composed of a number of horny bells which fit 

 into each other. The oldest or terminal bell is 

 in reality the horny covering of the tip of the 

 tail, and with each moult or shedding of the 

 skin the youngest bell becomes loose, but is held 

 by the new covering which has been developed 

 in the meantime. There is thus produced an 

 ever-increasing number of loosely-jointed bells, 



an( j then mogt Qr all the 



FIG 175 Rattle of 

 Rattle -'Snake. 

 (From White's His- off, probably when they are worn out, and a 



new set is gradually developed. Eattles with 

 a dozen bells are, for instance, very rare. Tbey naturally 

 increase in bulk with the age of the snake, but the number of 

 joints is no indication of the snake's age. 



CROTALINAE. 



Fid. 176. Map showing the distribution of the Sub-Family Crotalinae. 



Pit- Vipers have a very wide distribution. They are divided 

 into four genera with about sixty species. Eattle-Snakes are 



1 J. Linn. Soc. xxviii. 



