VIPERINE GROUP. 



239 



which in a perfect rattle remains at the tip, the various rings, which may reach to 

 twenty or more in number, being gradually interpolated between this and the 

 scaly portion of the tail. More or less symmetrical in form, the rattle is composed 

 of hollow, horny rings, somewhat like quill in substance, which are interlocked 

 with one another, and are yet so elastic as to allow of a considerable amount of 

 motion between them. The various rings do not appear to be formed with any 

 regularity, sometimes several being added in a single year, while at other seasons 

 but one is developed ; neither does there seem to be any relation between the 

 growth of the rattle and the changing of the skin. That very large rattles must, 

 however, belong to old snakes, is obvious ; and that this is really the case is shown 

 by the circumstance that at the present day rattles with twenty rings are very 



COMMON RATTLE-SNAKE (J nat. size). 



seldom met with, since with the advance of cultivation it is only rarely that these 

 noxious reptiles are suffered to attain their full age. The body is thick, and, 

 for poisonous snakes, somewhat long; and the poison-glands attain very large 

 dimensions. 



Common Since the rattle-snakes are extremely variable in coloration, 



Rattle-Snake, reliance has to a great extent to be placed on the arrangement of the 

 shields covering the fore-part of the head in the discrimination of the species. In 

 the common rattle-snake (Crotalus durissus) of North America, the distinctive 

 character is the presence of only two pairs of large shields between the large 

 supraocular and rostral shields ; these paired shields being separated by a series 

 of small ones in the middle line. Of these shields, behind the large triangular 

 rostral comes the four-sided anterior pair, representing the anterior frontals, while 

 to each of the latter further back joins a larger oval shield which must be regarded 



