SREL Reprint #2331




Catalogue of American Amphibians and Reptiles.

Dorcas, M.E., J.W. Gibbons, and H.G. Dowling. 1998.


CONTENT. A single polytypic species, S. pygaea, is recognized.

DEFINITION. Seminatrix pygaea is a small black North American water snake with a bright red venter. The head is only slightly distinct from the cylindrical body. The eye is moderate in size (diameter about equal to eye-nostril distance) and the pupil is round. A maximum SVL length of 485 mm has been reported, but most adults are <400 mm. The tail is relatively short, making up 15-25% of TL.

Scutellation is relatively unmodified. The head has the nine typical caenophidian crown scutes. The only indication of aquatic adaptation is the slight narrowing of the anterior ends of the internasals. The rostral is rounded, the nasal single, and a loreal is present. The oculars usually are 1 + 2 and the temporals usually 1 + 2 or 1 + 1. The supralabials generally number 8, the infralabials 9. Two pairs of subequal genials are present. The dorsal scales are smooth, rounded, and without apical pits. The lower rows are widest and they become progressively narrower dorsally. Dorsal scales are smooth and usually in 17 rows at the neck and midbody, but in 15 rows just anterior to the vent, the reduction occurring by loss of the fourth row on either side. A few individuals have 19 rows on the neck (also reduced by loss of the fourth dorsal scale row). Dorsal scales superior to the vent are usually keeled in males. Some scales may appear keeled under superficial examination due to a pale line running longitudinally through the center of the scale. Ventrals range from 112-134, the anal scute (cloacal scute) is generally divided, and 35-56 paired subcausals are present, with the males having about 10 more than do females.

Teeth are found on all normally dentigerous bones: maxillary teeth number 19-21, increasing slightly in length posteriorly; palatine, 10-12; pterygoid, 17-18; dentary, 20-21, all subequal in length (Dowling 1950).

The presence of recurved hypapophyses on all body vertebrae, spinose and acalyculate hemipenes with enlarged basal hooks and a simple centripetal sulcus, together with reduction of dorsal scale rows by loss of a mid-lateral row, all serve to clearly identify Seminatrix pygaea as a member of the American subfamily Thamnophiinae (Natricidae) as shown by Rossman and Eberle (1977). This placement has been corroborated by electrophoretic (Dessauer et al. 1987, Dowling et al. 1996) and immunological (Dowling et al. 1983) studies.

SREL Reprint #2331

Dorcas, M.E., J.W. Gibbons, and H.G. Dowling. 1998. Seminatrix Cope, black swamp snake. Catalogue of American Amphibians and Reptiles 679.1-679.5.

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