Definition of mountain beaver
: a bulky fossorial nocturnal rodent (Aplodontia rufa) of the uplands of the Pacific coast of North America having small ears and eyes and a rudimentary tail and superficially resembling the ground squirrels to which it is actually but distantly related being the sole recent survivor of a once extensive group (Aplodontoidea of the Sciuromorpha), of primitive rodents that first appeared in the Paleocene
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