Captains Lewis and Clark obtained some vague information about this animal, which is given in their journal of travel across the Rocky Mountains, in 1804. All they say of its habits is, ‘that it climbs trees, and digs like a squirrel.’ They obtained no specimen of the animal, but saw, probably, robes made of the skins. It was subsequently described by Rafinesque, and by him named _Anysonyx rufa_, and by Harlan _Arctomys rufa_. In 1829 Sir John Richardson obtained a specimen, and, after a careful anatomical examination, this eminent naturalist determined it to be a new genus, and renamed it, generically and specifically. The generic name (_Aplodontia_) is founded on its having rootless molars, or grinding teeth—_aploos_, simple; _odons_, a tooth. It belongs to the sub-family _Castorinæ_, dental formula 2 00 55 ——————— 22. 2 00 44 _Sp. ch._—Size, that of a musk-rat; tail very short, barely visible; colour, glossy blackish-brown. Male, length about 14 inches; female resembling the male, but smaller. The fur is dense and woolly, with long bristly hairs, thickly interspersed; the short fur is bluish-gray at the base, the ends of the hairs being tipped with reddish-brown; the bristles are black, and when smooth give a lustrous appearance to the fur. The eyes are very small, and placed about midway between the nose and the ear. The whiskers, stiff and bristly, are much longer than the head, and dark grey. The ears are covered on both sides with fine soft hair, rounded and very short, and not unlike the human ear.