82 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



Variation. — After exainininji; lumdreds of specimens I believe it 

 to be unquestionable that we have to deal here' with a single form 

 which is sharply defined from the other garter-snakes of the prairie- 

 plains region. Generally speaking, it may be defined as having 21- 

 19-17, 19-21-19-17, or even 19-17 scale rows, 7 (8) supralabials, 142 

 to 176 ventrals, 57 to <S7 subcaudals, and the lateral stripe on the 

 third and fourth rows of dorsal scales, but while this description will 

 serve very well for analytical purposes, it gives no clue to the rela- 

 tionships, and we have to examine the variations in detail. For the 

 sake of unity of treatment, as well as for the fact that this character 

 is less variable than the others, the number of dorsal scale rows will 

 first be examined. The number (maximum) for radix has generally 

 been given as 21, although the type had 19. Brown (1901) gives 

 the number as usually 21, but occasionally 19, and Cope (1892, 651) 

 states that the only specimen which he had examined with 19 rows 



21-19-17 



19-21-19-17 



19-17 



17-19-17 



Chicaco, Racine, 



Illinois. V.isccnsir.. 



Fig. 26.— Diagram showing the variation in the dorsal scale formula in Thamnophis radix. 



Fort Collins, Poabody, Ruthven, St. Louie, 

 Colorado. Kaneaa. lo»a, Missouri. 



was the type An examination of the following diagram (fig. 26) 

 will show the actual state of affairs. 



This table shows clearly that radix is not characterized by a single 

 dorsal scale formula in any part of its range, but the formula 19-21- 

 19-17 occurs the most frequently in all localities. While, however, 

 the range of variation is extensive, and the arithmetical mean re- 

 mains nearest to the formula 19-21-19-17, we believe that the dia- 

 gram is correct in indicating as it does that there is a tendency toward 

 a larger number of rows in the more western localities. This is 

 apparently shown both by the displacement of the mean from the 

 formula 21-19-17 toward or below the formula 19-21-19-17, in the 

 eastern localities, as well as by the lowering of the minimum number 

 toward the formula 19-17 or below, in the same direction." 



In plotting this diagram we have only used the localities from which 

 we have the largest number of specimens, and still the series from the 



« This decrease in the number of rows in eastern localities explains how the type of 

 ,the species (which came from Racine, Wisconsin) came to have 19 scale rows. 



