VARIATIONS OF GARTER-SlSrAKES. 



59 



Hahits and habitat relations. — Rarely is it possible to obtain a suf- 

 ficient number of records to map in detail the range of any form, 

 but if we know its preferred habitat we can, in doubtful regions, 

 determine more or less accurately the probable extent of its range 

 by the limits of the environmental conditions with which it is usually 

 associated. In the present case such data would be of great value, 

 but unfortunately, as far as we have been able to find, there is prac- 

 tically nothing recorded on the habits of marcianus, with the excep- 

 tion of the general conditions of the region it inhabits. I have 

 observed elsewhere (Ruthven, 1907, 589) that it is occasionally at 

 least found in the vicinity of streams, which is in harmony with the 

 known habits of the other arid region forms in the genus (fig. 20). 



Fig. 20.— Santa Cruz River at Tucson, Arizona. Thamnophis marcianus has been found 



HERE. 



Range. — The region inhabited by marcianus includes the Proplateau 

 region of southern Arizona, New Mexico, and Texas, the central part 

 of the latter State, and northern ISIexico. We have already briefly 

 discussed the geographic and climatic conditions of the Proplateau 

 region, so there remains to be considered onl}" the environmental 

 conditions in the Texas portion of the range of this form. This 

 region may for convenience be divided into two physiographic regions; 

 the prairie-plains region, situated roughh^ to the east of the Pecos 

 River, north of the thirtieth parallel and west of the ninety-eighth 

 meridian, and the Rio Grande plain, which occupies the triangular 

 area between the prairie-plains, Rio Grande River, and ninety-eighth 



