Source : Webster's Revised Unabridged Dictionary (1913)

Water mouse \Wa"ter mouse`\ (Zo["o]l.)
   Any one of several species of mice belonging to the genus
   {Hydromys}, native of Australia and Tasmania. Their hind legs
   are strong and their toes partially webbed. They live on the
   borders of streams, and swim well. They are remarkable as
   being the only rodents found in Australia.