186 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



number of specimens is needed from the latter region before this 

 point can be definitely demonstrated. 



Afjinities. — The only apparent near relative of sirtalis is imrietalis. 

 I have already discussed tliis relationship. It should be pointed 

 out, however, that the fact that typical specimens of sirtalis, from 

 Indiana to western Pennsylvania, have frequently a small amount of 

 red pigment on the sides, also points toward a relationship between 

 these two forms. Brown's (1904, 470-471) ingenious suggestion that 

 sirtalis is related to radix through hutleri has little to recommend it, 

 for from my point of view there is no evidence from the scutellation 

 that sirtalis is at all nearly related to hutleri, although the general 

 similarity in the scutellation and the intermediate position of the lat- 

 eral stripe in the latter has been occasionally seized upon as indicative 

 of such an affinity, while there is no evidence either in the position of 

 the lateral stripe, scutellation, or geographic distribution that it is 

 anything but distantly related to radix. To establish a relationsliip 

 with the forms of the Radix group one must entirely ignore the evi- 



concinnus parietalis — sirtalis 



eques 



sumichrasti 



Fig. 82.— PH'i'LOGENETIC DEVELOPMENT GF TOE SiRTALIS GROUP. 



dence of the lateral stripe and the geographic probabilities, as sirtalis 

 entirely overlaps the range of hutleri, and in part the range of radix, 

 without showing the slightest modification in any character that 

 would point toward any close relationship with either form. 



CONCLUSION. 



Of the four groups into which I believe the genus Thamnophis is 

 differentiated, the Sirtalis group is without doubt the least diver- 

 sified. This is shown by the fact that most herpetologists have been 

 willing to give to its members but subspecific rank, owing to their 

 evident intergrading, and by the further fact that variation, geo- 

 graphic or otherwise, is very slight. If we grant the unity of the 

 group as here described (fig. 82) the geographic differences may be 

 outlined as follows : 



1 . The dorsal scale rows are 19-17 in most forms, the only departure 

 from this formula being in southern Mexico, where it becomes 17-15. 



