REPTILES 223 



labials 12 to 16; ventrals about 175; subcaudals undivided: 15 species, in 

 North and South America, 12 in the United States, all very dangerous 

 snakes. The young rattler has no rattle, but a soft horny button at 

 the end of its tail. During its first season it adds one horny ring to the 

 button. In the succeeding seasons it usually adds 3 rings a year, the 

 terminal rings usually becoming lost as the snake gets older. 



Key to the United States Species of Crotalus 



ai Supraocular horned C. cerastes. 



a 2 Supraocular not horned. 



bi Eye separated from labials by i row of scales C. triseriatus. 



hz Eye separated from labials by 2 to 5 rows of scales. 



Ci Upper preocular small, higher than wide C. lepidiis. 



C2 Upper preocular large, much wider than high 



di Pattern of short cross bands of white C. willardi. 



6.2 Pattern not of white cross bands. 



Ci Dorsal bands enclosing paired light spots; tail black. C. mohssus. 

 e2 Dorsal bands not enclosing paired light spots. 



fi Dorsal pattern of dark chevrons; tail usually 



black C. horrid us. 



{2 Dorsal pattern of more or less squarish spots or 



straight cross bands. 



gi Large snakes with dorsal diamond markings; 



a light line from back of eye reaches scale 



row above labials at least 2 scales anterior to 



angle of jaw. 



hi North CaroHna to Louisiana C. adamanleus. 



ho Texas and Oklahoma to southeastern 



Cahfornia C. atro.x. 



ha Southwestern California C. ruber. 



g2 A light line from back of eye (if present) 

 reaches scale above labials only at angle of 

 mouth, or not at all; pattern of squarish 

 blotches or cross bands, 

 hi Squarish blotches present, at least anter- 

 iorly, 

 ii Two scales on snout between nasals; 2 



scales between supraoculars C. scutulatus. 



\2 Four scales on snout between nasals; 4 



to 6 scales between supraoculars C. conjiuentus. 



h2 Many narrow cross bars present C. tigris. 



C. horridush. Common rattlesnake (Fig. 127). Length 1,000 mm.; 

 tail 130 mm.; greatest length 1,750 mm.; color yellowish brown, with 

 about 22 wide, dark brown, more or less irregular cross bands, each of 

 which is often broken into 3 separate irregular blotches; abdomen 

 yellowish, with black spots; melanistic coloring not uncommon; tail 



