42 VENOMOUS SNAKES AND THE PHENOMENA OP THEIR VENOMS 



It is a snake of swampy localities, and eats cold-blooded animals, unlike Crotalus; 

 in this and in the presence of the large shields on the head the pigmy rattlesnake 

 resembles the copperhead or water-moccasin. It can be kept in captivity, as it 

 readily becomes tame and takes food. 



Sistrurus catenatus var. edwardii. 



Coloration much paler and more yellowish than the typical form. Habitat, 

 from Ohio to Nebraska, Minnesota, Wisconsin, and Michigan, and even some 

 districts in Canada and south to Mexican borders. The Edwards massasauga 

 extends farther south than the regular massasauga. 



Sistrurus ravus. 



General coloration yellowish-brown with a dorsal series of dark-brownish blotches, 

 and a series of crossbars; belly yellowish with brown spots. Length about 7 

 inches. Vera Cruz, Mexico. 



Genus CROTALUS Linnaeus. (Plate 15, D, E.) 



This genus is provided with the rattle as in the Sistrurus, but may be distinguished 

 from the latter by the scutellation of the head; this being small and granular. A 

 few varieties may be covered with some enlarged shields in the front of the eye. 

 The genus comprises 15 distinct species and several varieties; n of these species 

 and 2 varieties are found in the United States. The rattle consists of a number 

 of thin, but dry and more or less elastic, hollow, horny cones of successive sizes. 

 These horny cones have a stricture in the middle and are divided into an upper 

 smaller cone and a lower larger cone. The base of the lower cone is inwardly 

 turned and its diameter is smaller than that of the smaller upper section of the 

 next proximal cone, which is constructed in a similar manner. Thus the upper 

 globular portion of the horny cone serves to hold the next cone by means of the 

 inwardly turned edge of the latter. Of course the diameter of the base opening of 

 the second cone is longer than that of the strangulated part of the first cone, but 

 shorter than that of the upper globular part of the latter. The articulation of the 

 rattle-cones is very loose and a slight shaking is sufficient to cause the well-known 

 sound. 



The number of the cones or "buttons" does not agree with the number 

 of years the snake has lived, as is commonly believed, but increases irregularly irre- 

 spective of the age of the snake; it seldom surpasses 18 to 20 cones. 



Rattlesnakes live in all kinds of ground, but prefer rocky regions, where they 

 have abundant places of concealment. They are not of quick movement and do 

 not bite quickly unless trodden upon or attacked. It is hard to induce them to 

 take food when in captivity. 



I. A CHAIN OF LARGE, DARK, PALE-BORDERED RHOMBS OR "DIAMONDS." 1 



(a) Diamond markings closed on sides : 



Dark olive; rhombs with yellow borders. (Southeastern United States) . Crotalus adamanteus- 

 Grayish; rhombs with whitish borders. (Texas to southern California) . . . .Crotalus atrox. 

 Reddish; rhombs with whitish borders. (Southern and lower California) 



Crotalus atrox var. ruber. 

 Dull white or pinkish, with very obscure, rhomb-like markings. (Southern 



California, lower California, and Arizona) Crotalus mitchelli. 



(V) Diamond markings narrowly open at sides and continued downward as narrow bands : 



Yellow or greenish. Two paler blotches within each rhomb. (Arizona, New 



Mexico, and Mexico) Crotalus molossus. 



1 The South and Central American species are not in this key. 



