186 BULLETIN 18 7, UNITED STATES NATIONAL MUSEUM 



upper preocular frequently split transversely ; scale rows generally 



23 medially U 



Fifth supralabial below posterior edge of orbit ; pattern of small, paired 

 dorsal spots, sometimes fused medially ; upper preocular seldom di- 

 vided transversely; scale rows 21 medially 12 



11. Body pattern of relatively large, subquadrate or four-parted spots, usually 



less than 40 in number triseriatus triseriatus 



Body pattern of small circular or elliptical, single median spots, gen- 

 erally 40 or more in number ^ triseriatus anahuacus 



12. Ventrals more than 150; general coloration usually gray; dorsal .spots 



usually separate, in pairs triseriatus pricei 



Ventrals less than 150 ; general coloration predominantly brown ; dor- 

 sal pairs of spots often connected medially triseriatus miquihuanus 



13. Top of head anterior to supraoculars and frontal region covered by sym- 



metrical plates recognizable as paired internasals and prefrontals 

 (or canthals), the total number usually four or six and rarely exceed- 

 ing eight, if subdivision has occurred ; loreals two or moi*e ; tail entirely 

 black or dark gray with dark rings faintly visible and not in shai'p 



contrast with ground color 14 



Top of head anterior to supraoculars and frontal region covered by numer- 

 ous small scales in irregular arrangement; tail rings usually distinct* 

 and contrasting with ground color 18 



14. A pair of dark vertebral stripes one to three scales wide on neck 



and extending one and one-half 'to five head lengths poste- 

 riorly durissus durissus 



No paired dark vertebral stripes on neck ; or if present, ill-defined and 

 not extending as much as one and one-half head lengths before meet- 

 ing first dorsal blotches 15 



15. Tail usually entirely black in adults ; muzzle and supraoculars uni- 



formly dark, no sharply defined mai'kings on snout 16 



Tail gray, with darker cross bands usually evident; muzzle and supra- 

 oculars not uniformly dark, sometimes a distinct pattern on snout 17 



16. Scale rows usually 27 ; body pattern of large dark brown or black rhombs, 



most of which are expanded at the sides and extended down sides as 

 cross bands ; blotches bordered by a row of light unicolored scales and 

 often enclosing patches of light scales; ground color yellowish olive, 



light olive-green, or yellowish gray molossus molossus 



Scale rows usually 25 ; body pattern of irregular diamond-shaped rhombs, 

 closed laterally by light borders of unicolored scales, not extending 

 down sides except posteriorly ; ground color light brownish olive or 

 yellowish olive, often obscured by black posteriorly molossus nigrescens 



17. Body pattern of irregular diamond-shaped reddish brown rhombs, little 



lighter in the centers, and with light borders one scale wide, usu- 

 ally notched on posterior edges ; dorsal head pattern usually in- 

 distinct (both head and body pattern sometimes obsolete in large 

 adults) ; spinal ridge prominent; scales tuberculate, especially poste- 

 riorly basiliscus 



1 In some specimens the pattern is indistinct ; the general color may be very light or 

 very dark. 



^ An exception to this is found in enyo in which the tail rings are indistinct and incom- 

 plete, but this course may be followed safely in the majority of cases if the scales anterior 

 to the supraoculars are not in symmetrical arrangement and are more than 11 in number. 



