No. 433.] SNAKES OF SOUTHERN MICHIGAN 



19 



there appears that the range of variability in the length of tail 

 is over 10 per cent in sip e don and only a little over 5 per cent 

 in etythrogaster. We have already seen that sipedon is much 

 more common and much less restricted in its range than ery- 

 throgaster, and since it is clearly much more variable, these facts 

 serve as an excellent illustration of the generally accepted belief 

 that common and wide-ranging species are the most variable. 



Table VII. 









1 



j 

 1 





1 - 



H 



j 

 I 





■ ■ 



] 



f 







1 



1 









>.-. 



8 

 | 



h 





js 



II 



1 







8 







s 

 ? 







5 



<*% 







74% 







96% 





79% 







$ 



9 













76% 







81% 



7°% 





!I 





49% 



The Systematic Position of Erythrogaster. 

 With these facts before us, we may well consider what light 

 they throw on the real relationship of the red-bellied black 

 snake to the common water snake. The table on the follow- 

 ing page will help to set before us, so that they may be readily 

 grasped, the points of resemblance and difference between 

 the two. 



In the relative length of head and tail, in the number of scale 

 rows, and in the number and arrangement of labials, there is 

 evident agreement between the two forms ; but, on the other 

 hand, erythrogaster is a larger snake than sipedon, the male 

 especially averaging 50 per cent more (see Table II), the eye 

 is very much larger, the gastrosteges are more numerous, the 

 urosteges slightly more numerous, the color is totally different, 

 the odor is distinguishably different, and the percentage of 

 variability is very much less. In addition to these characters, 

 certain peculiarities of habits and habitat help to distinguish 



