Thamnophis has an anal scale that is not divided and Nerodia has an anal scal that is divided.
A, B & C - Female tail; ventral and lateral views. The tail base enlargement noticeable in some females (see B) appear to be male glands, but note that the tail tapers rapidly
D & E - Male tail; ventral and lateral views. Tail base is thick, and this thickness extends for some distance beyond the vent.
Illustration and description by kind permission of Mr Robert J. Riches, author of 'Breeding Snakes in Captivity', 1976
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