CHECKLIST OF THE SNAKES OF MEXICO 187 



Body pattern of diamond-shaped brownish blacli rhombs, with light 

 brown centers and yellowisli white borders; posterior edges of 

 blotches fairly even, not notched ; dorsal head pattern distinct in 

 adults and young, a conspicuous dark bar across prefrontal region ; 

 spinal ridge absent or inconspicuous ; scale tubercules not pro- 

 nounced durissus totonacus 



18. Prenasals in contact with rostral ; upper preoculars usually not divided 20 



Prenasals usually separated from rostral by small scales or granules ; 



upper preoculars often divided, horizontally, vertically, or both ; ros- 

 tral usually wider than high ; a pattern of dorsal blotches essentially 

 comprising aggregations of punctations 19 



19. Head smaller; length of head contained in body le^^gth (adults) more 



than 24 times ; original rattle button, if present, more than 7.5 mm. 

 wide ; a pattern of dark gray or brown, punctated blotches on a gray 



or tan background mitcliellii mitchellil 



Head larger; length of head contained in body length (adults) less 

 than 24 times ; original rattle button, if present, less than 7.5 mm. 

 wide ; a pattern of red, gray, brown, or black, punctated blotches 

 on a cream, tan, buff, gray, pink, salmon, fawn, or brown back- 

 ground ; often with posterior black tips on some dorsal scales be- 

 tween blotches mitchellii pyrrhus 



20. Ground color of tail light ash gray or white, with distinct black rings, 



both in sharp contrast with the posterior body color, which may be 



gray, dark gray, cream, pink, ted, reddish brown, or olive-brown 21 



Ground color of tail essentially the same as that of body posteriorly ; 

 tail rings not in sharp contrast with color of body, or the proximal 

 ones of same color as body blotches and the distal ones darker 2ft 



21. Dark tail rings approximately equal in width to light interspaces ; post- 



ocular light stripe, if present, intersects supralabials one to three 

 scales anterior to angle of mouth ; minimum scales between supra- 

 oculars (counted in one transverse row) three or more; no definite 

 line of demarcation between scales of frontal and prefrontal areas ; 



proximal segment of rattle usually all black 22 



Dark tail rings narrower than light interspaces; postocular light stripe, 

 if present, passing backward above angle of mouth ; minimum scales 

 between supraoculars rarely more than 2; a definite division line or 

 suture between scales of frontal and prefrontal areas; lower half of 

 proximal segment of rattle light in color, upper half black ; a pattern 

 of brown hexagons or diamonds on a green, olive-green, or brown back- 

 ground ; light scales bordering dorsal blotches unicolor, the edges of the 

 blotches usually following the scales and not cutting them. 



scutulatus scutulatus. 



22. First infralabials usually not divided transversely (or if divided, general 



coloration not pink or reddish brown) ; general coloration cream, buff, 

 gray, or grayish brown (sometimes pink or red in southern Arizona 

 and southwestern New Mexico) ; dark punctations conspicuous 



in markings 23 



First infralabials usually divided transversely ; coloration pink, red, brick 

 red, reddish brown, or olive-brown; dark punctations in markings 

 weakly in evidence or absent 24 



23. Upper preocular usually not in contact with postnasal, and no upper 



loreal present; head smaller in proportion to body; pattern of dark 

 brown, punctated diamonds with lighter centei's on a gray back- 



