water mouse

Wiktionary

1. n. zoology Any of several species of Australian mouse of the genus Hydromys, native to Australia, with strong hind legs and partially webbed toes.

GNU Webster's 1913

1. (Zoöl.) 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.