POISONOUS SNAKES OF NORTH AMERICA. 



423 



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HEAD OF CROTALUS, FROM SIDE. 



ilnfralabials: I loreal ; P lower loreal; I'' upper loreal; )i ' auterior nasal; a' posterior nasal; p 

 preocular; r rostral ; « siipralabials; s' 1st supralabial; s 2 2<1 supralabial ; -o Supraocular. 



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Pig. 56, HEAD OF CROTALUS MITCHELLII FROM FRONT. 



FRONT VIEW OF PRECEPING FIGURE. (See explanation of Fig. 55.) 



S!i)W2Jsis of the impedes of Croialus occurring in Xorih America north of Mexico. 



«' Auterior unsal in coutact with rostral (tigs. 55, 56). 

 b' Upper preocular uot divided vertically (tigs. 55, 56). 

 c' External border of supraocular not produced into a lioru-like process (tigs. 

 55, 56). 

 d^ Dark spots ou back with two symmetrical light spots, one ou each side of 



median line (tig. 51) C. moJossHS. 



d- Dark spots ou back, solid, or with ouly one median liglit spot. 



c' Dorsal pattern, consisting of dark chevron bands (fig. 52) C. horridus. 



e- Dorsal pattern, consisting of more or less squarish spots or straight cross 

 bands (figs. 53, 54). 

 /' Rostral at least as high as wide (fig. 63). 



g^ Light jio.stsuperciliary line reaches the second scale row above com- 

 missure at least two scales auterior to angle of mouth (tig. 58). 

 /i' A well-defined vertical white line on first labial and auterior nasal, 

 occupying the posterior half of the latter (fig. 58) . . ('. adamantens. 

 k' No white line on first labial and nasal, which are uniform in color 

 and more or less dusted over with minute blackish dots. 



i' Predominant color grayish-brown C. atrox. 



i- Predominant color reddish-brown C. atrox ruber. 



g- Light postsuperciliary line reaches the second scale row above commis- 

 sure at corner of mouth or not at all (figs. 59, 60). 

 /i' Light postsuperciliary line one scale wide; dark postocular patch 



starts from below anterior angle of eye (fig. .59) C. confluentus. 



A" Light postsuperciliary line two scales wide; dark postocular patch 



starts from below center of eye (fig. 60) <'. lucifer. 



f- Rostral wider than high (fig. 61) C tigris. 



c- External border of supraocular produced into a horn-like i)rocess 



(fig. 66) C. cerastes. 



b- Upper preocular divide<l vertically (fig. 62) ' C. lepidus. 



a' Anterior nasal separated from rostral by scales (fig. 63). 



6' Predominant color whitish C. mitcluUii. 



b- Predominant color red <'■ mitchelUi pyrrhus. 



