52 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



Piiebla. While a similar reduction is indicated in my material from 

 southern Arizona, the series upon which the computations are based 

 is too small to be relied upon as representative. 



The variation in the number of subcaudal plates is very similar to 

 that of the ventral plates, but, owing to the much wider range of 

 both sexual and individual variations, the geographic is even more 

 liable to be obscured. In general the males may be said to have from 

 none to about 10 urosteges more than the females, while the individual 

 variation in the males may be as high as 12 and in the females 8. 



Id 



li^ 



5 



^=^c 



\ 



^6 



ji. 



26 



;C j^ 



92 



84 



76 



65 



60 



Fig. 18.— Diagram showing the variation in the number of subcaudal scutes in Tham- 



NOPins megalops. 



If we take only those localities from which seven or more specimens 

 have been examined and examine the range of variation in the two 

 sexes, there is shown that in Chihuahua and Durango the variation 

 is from 75 to 87 (males) and 66 to 74 (females), while at the City of 

 Mexico and Puebla it is 67 to 79 (males) and 61 to 67 (females), thus 

 seeming to reveal a slight geographic variation. If males and 

 females be combined in approximately equal numbers (fig. 18), an 

 average of 76.3 is given for Chihuahua and 66.6 for Mexico City, 

 which shows a shght decrease in harmony with the reduction in 



