VARIATIONS OF GARTER-SNAKES. 



53 



scutellation, observed in the case of the scale rows, labials, and 

 gastrosteges. 



To summarize, then, there is exhibited in this form a decided 

 tendency toward a decrease in the number of scale rows toward 

 the southiern part of the Mexican plateau, which is correlated with 

 an apparent decrease in the number of labial, ventral, and subcaudal 

 plates, and the locality records suggest that a similar reduction 

 in the number of scale rows and ventrals exist in southern Arizona. 

 In general I think that it can be safely affirmed that this reduction 

 in scutellation to the southward is an evidence of dwarfing in this form, 

 although it is impossible to prove it, of course, by measurements. 



The proportionate tail length of specimens from the various 

 localities has been examined and seems to be quite constant through- 

 out the entire province, although an apparent geographic variation 

 in this trait seems to be demonstrable. In Chihuahua it varies 

 from .24 to .27 of the total length in the males, and in the females 

 from .20 to .23, while about the cities of Mexico and Puebla it (hops 



.26 



./.8 



\l 



7 



6' 



15 



<^ 



.n 



Fig. 10. 



-Diagram showing the variation in the proportionate tail length in Thamno- 

 phis megalops. 



to .20 to .22 (females) and .21 to .24 (males). When these are 

 averaged in equal numbers and plotted (fig. 19), a slight decrease in 

 the average length of the tail to the southward is evident. It is 

 true that this reduction only amounts to .03, but it must be noted 

 that the sexual and the individual variations have been excluded in 

 large part, while a reduction approximately equal to the extremes 

 of variation in either sex in one locality is still indicated. Tliis 

 decrease in the length of tail should not be confused with the de- 

 crease in the number of dorsal rows, labials, or gastrosteges, of 

 which it is entirely independent, but it possibly explains in part the 

 reduction in the number of subcaudals. 



When the scutellation and tail length is plotted, the value' of the 

 divisions that have been made of the group on the basis of these 

 characters is at once revealed. In 1860 Kennicott (1860, 330) 

 tlescribed a specimen labeled ''Tucson and Santa Magde" (No. 965, 

 U.S.N.M.), collected by Major Emory and A. Schott, which had a 



33553— Bull. 61-08 5 



