VARIATIONS OF GARTER-SNAKES. 43 



the region west of the Rocky Mountains, only occurring east of these 

 mountains on the liigh plateaus; the Sirtalis group occupies the 

 plateau region in Guatemala and Mexico, and in addition practically 

 all of the United States and southern Canada, from the Atlantic to 

 the Pacific. The range of each of these groups, as will be shown 

 later, is made up of the ranges of the component forms placed side 

 by side. 



The evidence seems to be quite conclusive, therefore, that these 

 groups are assemblages of directly related forms, and it is necessary 

 now to consider in detail the forms comprising these groups in order 

 to determine the directions of their relationships. If tliis detailed 

 examination bears out the evidence that the groups. are genetic, and 

 reveals the direction of the affinities within the groups, much light 

 should be thrown upon the origin and history of these snakes. 



A key is given by which the forms may l)e recognized. 



Key to the forms of the genus Thamnophis. 



a'. Lateral stripe anteriorly upon the third and fourth rows. 

 6'. Tail generally more than .27 of the total length, 

 c'. Supralabials usually 8. 



rf'. Tail between .25 and .35 of the total length proximus, p. 98. 



(Illinois to western Nebraska, southward in eastern Mexico to Central 

 America.) 



d-. Tail between .32 and .38 of the total length sackeni, p. 107. 



(Principally confined to Florida.) 



C-. Supralabials usually 7 savritus. p. 112. 



(United States east of the 87th parallel, and north of Florida.) 

 b-. Tail generally less than .27 of the total length. 



c'. Dorsal scale rows usually 21-19-17, supralabials 8, occasionally 9 



megalops, p. 44. 

 (Southern Arizona and New Mexico, the plateau of Mexico.) 



c'-. Dorsal scale rows usually 19-21-19-17, supralabials 7 or 8 radix, p. 70. 



(Great plains and prairie regions of central North America.) 



c^. Dorsal scale rows usually 19-17, supralabials 6 or 7 hutleri, p. 87. 



(Indiana, Ohio, southern Michigan, western Pennsylvania.) 



o^. Lateral stripe anteriorly upon the third row only marcianus, p. 58. 



(Central Texas, southern New Mexico and Arizona, northenr Mexico.) 

 a^. Lateral stripe upon the .second and third rows. 

 ¥. Dorsal scale rows usually 21-19-17 or 19-21-19-17. 

 c'. .Dorsal stripe usually absent. 



cP. Eye in contact with two supralabials hammondi, p. 133. 



(Southern California and northern Lower California.) 



d-. Eye in contact with a single supralabial anguslirostns, p. 120. 



(Northern part of the Mexican plateau, southern New Mexico and Arizona.) 



c~. Dorsal stripe usually present elegant, p. 138. 



(From the Piocky Mountains to the Sierra Neyada-Cascade Range, inclusive.) 

 b-. Dorsal scale rows usually 19-17 or less, 

 c'. Dorsal stripe usually present. 

 d\ Never more than 1 preocidar. 



e\ Supralabials usually 8 equcs, p. 158. 



(Guatemala to southern Arizona and New Mexico on the plateaus.) 

 e^. Supralabials usually 7. 



