Several 'tweets' replied privately with that question yesterday.

For any we't sider in the Pacific Northwest, this is one mammal you should at
least get to know if not appreciate for it is exclusive and endemic to the
Pacific Northwest and Cascades west from northern California to southern
British Columbia. It is also the only species representative of the unique
rodent order of mammals known as Aplodontiidae and goes by the common name
"Aplodontia" or "Mountain Beaver" (_Aplondontia rufa_). Adults may reach
12-17 inches in total length, or about the size of a small chunky house cat.
It looks like a tailless Muskrat with little beady black eyes and small
rounded ears. According to the Peterson Field Guide to Mammals, "it is
mostly active at night and makes extensive tunnels, runways, and burrows
beneath dense streamside vegetation." Peterson goes on to read: "In the wild
areas the Aplodontia is of little importance, but it can be a nuisance in
reforestation projects; also, it may raid truck gardens and cause general
damage by its persistent burrowing. Meat is strong and the hide is
worthless. It is an interesting element in the biological world, probably
its only asset".

Well! This last part isn't very nice and sounds a bit understated doesn't
it. Kevin Li mentioned that Kelly McAllister, one of our regularly
contributing members of the 'Tweeters' family, has written a paper on the
Aplodontia in Washington. So Kelly, if you're lurking about, maybe you'd
care to chime in. I would be interested in locating the paper at the very
least. Otherwise, I wouldn't know where to tell someone to go in search of
the reclusive Aplodontia / Mountain Beaver. Their sign is left all around in
many places around here. One site that comes to mind is the eastern most
soccer field at Lake Sammamish State Park (adjacent to the little pine grove
that was home for the Long-eared Owl two years ago). In the winter, there
are telltale mounds of dirt all over the field which in itself adds a new
dimension and challenge for all those little kids out there playing the game
of soccer.

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Richard Rowlett (Pterodroma at aol.com)
Bellevue (Eastgate), WA