50 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



occur. This variation is so slight that were it not for the considerable 

 number of specimens upon which the averages have been based it 

 might almost be ignored. However, it indicates, we believe, that 

 there is a geographical variation in these plates, correlated with the 

 decrease in the number of scale rows to the southward in Mexico. As 



8 



8 



il 11 



>c- ^ 



9c) 35 



j i U 



Fig. 15. 



we have before stated, the number of infralabial plates is more subject 

 to individual variation and requires a larger series than is here avail- 

 able to furnish the mean number for each locality. Fig. 16 will show 

 the amount of variation in the material examined. 







Fig. 16 — Diagram showing the variation in the number of infralabials in Thamnophis 



megalops. 



In regard to the ventral scutes a similar variation is to be observed, 

 but, as indicated above, the question of the sex of the specimens 

 examined must be carefully noted. In general it may be said that 

 there are in the males from to 17 more plates than in the females, 



