16 BULLETIN 18 7, UNITED STATES NATIONAL MUSEUM 



60. Scales keeled 61 



Scales smooth 73 



61. Anal divided 62 



Anal single 68 



62. Maximum scale row number, near middle of body, 17 or less 63 



Maximum scale row number, near middle of body, 19 or more 66 



63. Scale rows 15 at middle, 11 at posterior end of body LeptopMs (p. 90) 



Scale rows 17 at middle of body, 13 or 15 posteriorly 64 



64. No loreal ; prefrontal in contact with two or three labials ; head elon- 



gate, tapering Oxybelis (p. 101) 



A loreal ; prefrontal not in contact with labials ; head not notably 

 elongate 65 



65. One anterior temporal; seven supralabials Opheodrys ' (p. 101) 



Two anterior temporals; usually nine supralabials Drymobius (p. 56) 



66. Nasal scales enlarged, on top of snout, in contact or narrowly separated ; 



scales in 21-17 rows ; no pits on dorsal scales Tretanorhinus (p. 144) 



Nasal scales normal, not in contact ; apical pits present on dorsal 

 scales ; if scale rows 21 medially, 19 posteriorly 67 



67. One anterior temporal; scales strongly keeled; usually three post- 



oculars Natrix (p. 154) 



Two anterior temporals ; scales weakly keeled ; usually two post- 

 oculars Elaphe (p. 58) 



68. Rostral greatly enlarged, with free edges, separating internasals ; scale 



rows 19^17 Phyllorhynchus (p. 104) 



Rostral not or slightly enlarged, not with free edges; internasals in 

 contact; scale rows variable 69 



69. Two or more anterior temporals 70 



One anterior temporal 72 



70. Scale rows medially 17, posteriorly 15 Dendrophidion (p. 46) 



Scale rows more numerous 71 



71. Scale rows 23 medially, 15 posteriorly Pseustes (p. 114) 



Scale rows not less than 27 medially, 19 posteriorly Pituopliis (p. 105) 



72. Scale rows in even numbers, 18 at middle of body, 14 posteriorly ; very 



large tree snakes, with prominent black and yellow markings; not 



striped Spilotes (p. 131) 



Scale rows in odd numbers, 17 to 23 at middle of body, 15 (rarely 14) 

 to 17 posteriorly ; terrestrial snakes not over 3 feet in length, usually 

 striped or at least not brilliantly marked Thamnophis (p. 158) 



73. Anal divided 82 



Anal single 74 



74. Maximum scale rows 15 Dipsas ' (part) (p. 49) 



Maximum scale rows 17 or more 75 



75. Maximum scale rows 17, posteriorly 15 or 14 Drymarchon (p. 53) 



Maximum scale rows 19, posteriorly 17 or more 76 



76. Rostral greatly enlarged, with free edges, separating internasals ; scale 



rows 19-17 Phyllorhynchus (p. 104) 



Rostral not or less enlarged, not separating internasals; if enlarged, 

 scale rows 22 or more 77 



77. Internasals fused with prefrontals on each side Sjrmpholis (p. 134) 



Internasals and prefrontals distinct 78 



" Opheodnjs aestivua (Linnaeus) only. 

 ' Dipsas sanniolus only. 



