102 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



region, but it is significant that the records in the phiins region of 

 Texas are all on rivers that flow eastward into the Gulf of Mexico, 

 while the Kansas, Nebraska, and Iowa records are all in the vicinity 

 of tributaries of the Mississippi. This does not prove conclusively 

 that the form only occurs along streams in the grass-land regions, 

 but since, I believe, it will generally be found closely associated 

 with aquatic habitats even in the more humid and forested parts of 

 its range, it would be surprising if it had a wider distribution in the 

 more arid and treeless parts of the region in which it occurs. 



Variation. — Proximus exhibits such stability in most of its char- 

 acters that it should be comparatively easy to detect the nature of 

 such differences as do occur. The number of scale rows is inva- 

 riably 19-17 throughout the greater part of its range. Indeed, I 

 have observed but a single specimen with a dift'erent formula, a speci- 

 men (No. 755) from Orizaba, Veracruz, in the U. S. National Museum, 

 which has 17-19-17. It should be noted that this locality is toward 

 the southern limit of the known range of the form. 



While the superior labials are somewhat more variable (fig. 36), the 

 variations are still slight. The onlv variations observed are one 



8 



7 



)c- 



'n- 



~_-.^5 



30 



C) 



^- - - -^ ^ 



-^^ 



Mt. Carniel, St. Louis, 

 OIney, Missouri. 



Illinois. 



Ames, New Orleans, Kansas, 

 Iowa. Louisiana. Nebraska. 



Veracruz, 

 Yucatan. 



Fig. 36.— Diagram showing the variation in the number of supralabials in Thamnophis 



SAURITUS proximus. 



specimen from Progreso, Yucatan, and two from New Orleans, with 

 7 on both sides, and one with 7-8 from Belair, Louisiana, one speci- 

 men with 7 from St. Louis, Missouri, one with 7-8 and one with 7 

 from Olney, Illinois, and two with 7 and one with 7-8 from Mount 

 Carmel, Illinois. These are slight variations, but it is significant that 

 the tendency toward a decrease is only shown near the eastern and 

 southern limits of the range. In the case of the number of dorsal 

 scale rows and supralabials the variations are in all cases in the nature 

 of a reduction. In the number of inferior labials, however, we find 

 both an increase and decrease from the usual number, 10. The only 

 two variations observed are 9 and 11, and these occur so rarely and so 

 generally throughout the range that much larger series must be exam- 

 ined before we can hope to detect any geographic differences in this 

 character. 



I have stated that proxiTnus has from 150-179 ventral plates. It 

 is with reluctance that I give the table below (fig. 37), for I have not 

 enough specimens to plot the sexes in equal proportions and have 



