Page:Natural History (1848).djvu/225

Rh the toes to a greater or less degree connected by membrane: all the feet are five-toed.

The Beavers are constructed for a semi-aquatic life&#59; and though some of the aberrant species, as our own little Field Vole (Arvicola agrestis), do not, as far as we know, voluntarily take to the water, yet their fondness for damp situations, and their inability to resist a dry season, shew the family habit, though in a slight degree. They are confined to the Northern Hemisphere.

The single species of which this genus is composed has always possessed much interest, not only as yielding a medicinal drug, and a fur very extensively used in the formation of an indispensable article of dress, but principally as displaying an architectural instinct, very rare among Mammalia, though so common among Birds and Insects, many of which exercise it in a far superior degree. The Beaver is distinguished from the other genera of the Family, by having four molar teeth on each side, both above and below: by the hinder feet being strongly webbed, the membrane reaching beyond the bases of the claws: by having the second toe of each hind foot furnished with a double nail, an upper one corresponding to the others, and a lower one placed obliquely, with a sharp edge directed downwards: and above all by the peculiar structure of the tail. This organ is nearly half as long as the body, broad and oval, flattened above and below, but somewhat thick, composed of a gristly fat, and covered with an incrusted skin, which is