786 13. COLUBRIDA 
The color above is uniform light grayish, yellowish, or 
brownish olive, with more or less distinct blackish markings 
laterally upon the bases of many of the scales of the fourth 
and fifth, and sometimes of the first and eighth, rows. The 
lower surfaces are pale grayish yellow, the tips of the gas- 
trosteges being tinged with the color of the back. 
A second type of coloration, shown by certain melanistic 
specimens, has been described (Natrix celeno) but doubt- 
less is only an individual variation. In it the ground color 
both above and below is grayish black. A lighter stripe of 
bluish gray extends along the side from the neck to the tail. 
Anteriorly this stripe occupies the second, third, and part of 
the first, rows of scales, and sometimes shows irregular 
dorsal expansions or vertical bars separated by interspaces of 
the blackish ground color about one and a half or two scales 
wide. Posteriorly the lateral stripe usually becomes darker 
and more indistinct and may or may not involve the scales 
of the first row. The lower surfaces are grayish black often 
marbled with lighter. 
Some specimens are more or less intermediate between 
the two types of coloration. Some show, more or less dis- 
tinctly, seven longitudinal stripes, alternating light and dark, 
from head to tail. 
Length to anus... 432 540 575 620 682 687 
Length of tail__.___ 149 163 183 180 208 250 
Distribution —Natrix valida was first described by 
Robert Kennicott, in 1860, from a specimen collected in 
Durango, Mexico. It has since been found in other parts 
of the Mexican mainland. It probably does not occur in the 
United States. Professor Cope recorded a specimen (U. S. 
Nat. Mus. No. 4650) as having been collected in Rabeh 
Valley, Utah, but recent examination shows that this speci- 
men is a Thamnophis o. vagrans with a divided anal plate. 
