VARIATIONS OF GARTER-SNAKES. 



103 



had to ])ase my averages upon all of the specimens available, and I 

 am well aware that the sexual variation is suflicient to render the 

 averages very deceptive in such small series. If care is taken, how- 

 ever, not to attach too much importance to the averages, I believe 

 that the table indicates very plainly that the number of ventral plates 

 is quite constant over the North American portion of the range, and 

 that from the State of Veracruz southward, in Mexico, there is a 

 decrease in the number of scales in this series. This decrease in the 

 southern part of the range is shown not only by averages from the 

 different localities, but also by the fact that the smallest number of 

 ventral plates obsei'ved in one hundred specimens examined from the 

 United States is 158 (which occurs in but two specimens from New 

 Orleans), while both specimens from Yucatan, in which the scales 



182 

 174 

 166 

 15 8 



150 



H- 



,-^u 



30 



C) 



;i' 



18 



<) 



i\It. Carnu 



Olney, 



Illinois. 



Ames, 

 Iowa. 



Douglas, New Orleans, Perry, Texas. Veracruz, 



Kansas. Louisiana. Louisiana. Yucatan. 



Fig. 37 



-Diagram showing the variation in the number of ventral scutes in Thamnophis 

 sauritus proximus. 



have been counted, is as low as 150. Further than this the maximum 

 number for six specimens from the State of Veracruz is 161, which 

 is lower than any number observed in the United States, with the 

 exception of the two New Orleans specimens. I readily grant that 

 the series of specimens examined is too small to furnish exact evidence 

 of the amount of decrease, but I believe that the records do indicate 

 plainly that a reduction in the number of ventral plates actually 

 takes place in southern Mexico. There is no evidence, however, of 

 a decrease in the number of plates in this series toward the eastern 

 part of the range, comparable to that shown by the supralabials. 



As before mentioned the tail in proxiinus is comparatively long; 

 the proportionate length observed varies from .25-. 37. Although 

 this variation is considerable, the extremes are seldom reached and 

 the mean length of tail is about .29 or .30 throughout the range. 



