NO. 2179. 



VARIATION IN GARTER SNAKES— THOMPSON. 



547 



VARIATION IN NUMBER OF DORSAL SCALE ROWS. 



This species presents the widest range of variation in the number of 

 scale rows that is to be found in the group. There are at present 

 known in the genus eleven distinct scale formulae, ten of which have 

 been recorded in T. ordinoides. These ten and the frequency of 

 occurrence of the five combinations found in the San Francisco series 

 are as follows: 



Per cent. 



21-23-21-19-17 



21-19-17 12 



19-21-19-17 56 



19-21-19-17-15 8 



19-17 16 



19-17-15 4 



The influence of sex upon the variations in this character is clearly 

 demonstrated. Among the specimens that vary from what may be 

 assimied to be the normal coimt of 19-21-19-17, those having the 

 lower counts are prone to be males, and those with the higher coimts 

 are largely females. This mcrease in the number of scale rows in the 

 female is associated with the need for the increase in the diameter of 

 the abdominal cavity when carrying the young. 



19-17-15-13. 

 17-19-17-15. 



17-15 



15-17-15..-- 



Per cent. 



















Asymmetrical 4 



The data showing the gastrostege levels on the right and left side 

 of the body at which the added rows begin and the suppressed rows 

 end may be learned from the following tables: 



Scale Formula 21 — 19 — 17. 



This is next to the highest count recorded in the species. In this 

 series it occurs in 12 per cent of the specimens of which 16 per cent 

 are males. 



