spots may be present within the cross-bands.

The western subspecies, A. p. leucostoma, has a comparatively dull pattern in which the ground color does not invade the center of the cross-bands. In many instances the bands are outlined by white scales, as in the Mexican moccasin (this character is not so prominent in A. p. piscivorus because of the paler ground color). A large, dark blotch usually occurs at the base of the cross-band and may completely cross the ventral scales. The characteristic variations found in piscivorus are also present in leucostoma.

The number of bands is often difficult to count because of the dark color of some specimens. Gloyd and Conant (1943:168) reported averages of 12.5 (11 to 16) and 12.2 (10 to 16) in males and females, respectively, of leucostoma and ranges of 10 to 17 for males and 10 to 16 for females with averages of 13 in both sexes of piscivorus. On 20 specimens of leucostoma from Texas the average number of bands was 12.7 (11 to 15). If the number of bands differed on the two sides of an animal, the total number of the two sides was divided by two.

Scutellation

The scutellation of the cottonmouth closely resembles that of the other species of Agkistrodon. For example, the nine cephalic shields are characteristic of most species of Agkistrodon, as well as most other primitive crotalids and viperids, and most colubrids. Most individuals have an additional pair of large scales behind the parietals.

The numbers of postoculars, supralabials, and infralabials are variable. On either side the postoculars (three in most specimens) are reduced to two in some specimens. The supralabials (eight in most specimens) frequently vary (usually on one side only) from seven to nine. The number of infralabials is somewhat more variable than the number of supralabials, the usual number being 11, but 10 is also common; 8, 9, and 12 are more rare (Table 1). In 102 snakes in which these characters were examined, four different combinations of supralabials and seven combinations of infralabials were found. Both characters together yielded 16 combinations, considering only the actual number of scales and not taking into account the side of the head on which they occurred (Table 2). The combinations found in a brood of seven young from Houston, Texas, are shown in Table 3 to illustrate the variability of this character. Gloyd and Conant (1943:168) found a variation of 6 to 11 (8) and 7 to 9 (8) supralabials and 8 to 13 (11) and 8 to 12 (10.4) infralabials in samples of 301 leucostoma and 119 piscivorus, respectively (numbers in parentheses represent average). Also of interest is the variability of the scales themselves. In one instance a scale was found that had not completely divided. In another specimen the last supralabial and last infralabial were one scale that completely lined the angle of the jaw. Instances of one scale almost crowding out another were common. In still other instances one or two supralabials were divided horizontally into two scales. Individual variation rather than geographical variation occurs in these characters.

TABLE 1.—Frequency of Occurrence of Various Numbers of Supralabial and
Infralabial Scales in 102 Cottonmouths.

TABLE 2.—Numbers of Supralabials and Infralabials of 102 Cottonmouths.

The dorsal scales of cottonmouths are strongly keeled except that those of the two lower scale-rows on each side are weakly keeled. Also they are slightly larger than the others. Two apical pits are present on each dorsal scale. The shape of the scales and number of scale rows vary depending upon the position on the body.