38 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



region tend to be similarly colored, as Allen has pointed out a number 

 of times in mammals and birds, so that it is impossible to distinguish 

 them sharply on this basis. The following unquestionable tend- 

 encies may be noted: 



1. A marked increase in bright colors in the Pacific coast region 

 in Washington, and Oregon, and British Columbia. This is shown 

 by the presence of bright reds and greens (concinnus, ordinoides). 



2. An increase in the amount of black pigment at the expense 

 of the paler colors in the region just mentioned. This is especially 

 shown in the enlargement of the lateral spots to the obliteration of 

 the interspaces on the skin and a narrowing of the dorsal stripe (con- 

 cinnus, ordinoides). 



3. A tendency toward a paler ground color and lighter stripes 

 in the forms inhabiting western Texas, southern New Mexico, southern 

 Arizona, and northern Mexico (marcianus, eques). 



4. A tendency toward the production of red pigment on the Great 

 Plains (parietalis and occasionally radix). 



5. A tendency toward dark colors in the forest region of eastern 

 United States. This is shown by the prevailing brown ground 

 color in sauritus and butleri, and the dark greenish and bro\vnish 

 olives in sirtalis. 



An increased darkness of color thus occurs in the more humid 

 areas, while the paler colors are usually found in the more arid 

 regions; some well marked exceptions occur, however. Thus, in 

 the arid region of northern Mexico angustirostris and melanogaster, 

 which are very dark in color, occur with the paler megalo'ps and 

 eques, and in central Texas proximus is much darker than marcianus. 

 More detailed notes on these forms are needed before much can be 

 done by way of explanation, but it is significant that the dark forms 

 (melanogaster, angustirostris, and proximus) mentioned above as 

 occurring with pale ones in an arid region are known to be more 

 than usually aquatic in habits. We believe that it will be found 

 that the color in these snakes is not, as in mammals and birds, closely 

 correlated with the major environmental .complexes, owing to the 

 fact that they prefer damp situations, and are thus not exposed to 

 the same conditions throughout their range as terrestrial mammals 

 and birds. At any rate, owing to the great variability, color is one of 

 the least important characters that have been used iii diagnoses. 



As is well known, the characters that have been discussed are by 

 no means all that have been used by students of the group to define 

 species. They are, however, those which we have found to be of the 

 most importance. The number of temporals and comparative length 

 of fore and hind chin shields, while both of use in defining certain 

 species, are too variable or the peculiarities of too limited occurrence 

 to permit of their use in a search for general relationships. The 



