water mouse

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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