tropidonotus sipedon

from The Collaborative International Dictionary of English v.0.48

Water adder \Wa"ter ad"der\ (Zool.)
      (a) The water moccasin.
      (b) The common, harmless American water snake
          ({Tropidonotus sipedon}). See Illust. under {Water
          Snake}.
          [1913 Webster]

from The Collaborative International Dictionary of English v.0.48

Water snake \Wa"ter snake`\ (Zool.)
      (a) A common North American colubrine snake ({Tropidonotus
          sipedon}) which lives chiefly in the water.
      (b) Any species of snakes of the family {Homalopsidae},
          all of which are aquatic in their habits.
          [1913 Webster]

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