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How to Spot a Cottonmouth Snake
The Cottonmouth is the only poisonous water snake found in North America. It is a pit viper and senses movement with its pit in between its eyes. The Cottonmouth snake is found in three subspecies. The Western Cottonmouth, the Eastern Cottonmouth, and the Florida Cottonmouth have common characteristics and traits that distinguish them from nonvenomous water snakes.
http://www.ehow.com/how_2090701_spot-cottonmouth-snake.html?ref=fuel&utm_source=yahoo&utm_medium=ssp&utm_campaign=yssp_art
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Agkistrodon piscivorus leucostoma is a venomous pitviper subspecies[4] found in the south central United States. It is the smallest of the three subspecies tends to be darker in color.
Common names: western cottonmouth,[2] water moccasin, cottonmouth,[3] more.
http://en.wikipedia.org/wiki/Agkistrodon_piscivorus_leucostoma