VARIATIONS OF GARTER-SNAKES. 



83 



plains region are so small that our conclusions might with justice be 

 objected to on this ground. If we examine all of the specimens from 

 the region west of the 97th meridian, however, regardless of locality, 

 we find that there is not a single specimen in the lot that has less than 

 19-21-19-17 scale rows in the dorsal series, although about half have 

 the formula 21-19-17, while in but a small series from Racine 19-17 is 



8 



7 



9<> 



u ^: 



34 



H 5^ 



18 



^e 





Fig. 27 



Fort Collins, 

 Colorado. 



Pea body , 

 Kansas. 



Ruthven, 

 Iowa. 



St. Louis, 

 Missouri. 



Chicago, 

 Illinois. 



Racine, 

 Wisconsin. 



-Diagram showing the variation in the number of supralabials in Thamnophis 



RADIX. 



the common formula. I believe, therefore, that it is indisputable 

 that a decrease in the mean number of rows toward 19-21-19-17 

 rows or less takes place in the western part of the prairie region. 



The number of superior labials have been variously given as 7, 8, 

 and 7, occasionally 8. I have plotted the number for various 

 localities in the diagram, fig. 27, and an examination of this table 

 shows that the average number is verv close to 7 throughout the 



10 



8 



-IJ 



^\ 



34' 



:. 18 



~;;^ 

 -k)— 



— ^ 



^\ 



Fort Collins, 

 Colorado. 



Peal>ody, 

 Kansas. 



Ruthven, 

 Iowa. 



St. Louis, 

 Missouri. 



Chicago, 

 Illinois. 



Racine, 

 Wisconsin. 



Fig. 28.— Diagram showing the variation in the number of infralabials in Thamnophis 



RADIX. 



range. Indeed, so close does the mean approximate 7 that no geo- 

 graphical variation in the average number can be detected with the 

 series at present available. On the other hand, it should be pointed 

 out that it is only in the specimens from the prairie region that less 

 than 7 superior labials are found. Very similar conditions exist in 

 the case of the inferior labials. Thus, in the diagram, fig. 28, it will 



