182 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



believe that these averages are quite representative of the actual 

 conditions. 



The tail length and number of subcaudal plates also exhibit no 

 noticeable geographic differences. The extremes are: Subcaudals 

 54-74 (female), 62-84 (male); average for localities with more than 

 fifteen specimens, 64-70; tail length, .192 to .262; average of fifty- 

 four specimens from Washtenaw County, Michigan, .221. 



Percentage of 

 specimens. 



.9 8 

 .82 



.66 

 .50 

 .34 



.18 

 .02 



' 8-8 7-8 7-7 7-6 6-6 



Supralabials. 



Fig. 79.— Variation in the number of supralabials in 113 specimens of Thamnophis sirtali^ 

 from southeastern michigan. 



, Similarly but few of the color phases have distinct geographic 

 limits. About six of these forms have been described. 



(1) ordinatus Linnaeus. — This color phase is characterized only by 

 the absence of the dorsal stripe, but even so, if the character were 

 constant, some excuse might be had for considering it distinct from 

 sirtalis. As it is, however, the same brood may contain individuals 

 both of this and the normal phase, while among adults various 

 intermediate stages in the distinctness of the dorsal stripe occur, so 



