Mountain beavers are confined to the Pacific Coast and range from southern British Columbia to central California. The genus contains a single species of which Taylor (1918) recognized nine races. The principal habitat of the mountain beaver is clearings at the edge of coniferous forests. The animals are most abundant near springs, streams and damp places, although they are not aquatic. The tangled jungles of deciduous trees and shrubs that grow in the ravines and stream valleys of the Puget Sound area present optimum habitat. They occur also on hillsides, on logged-off land and along roadside clearings. In the mountains they occur in thickets and forests, always, in our experience, near streams. The most conspicuous evidence of the presence of mountain beavers is their burrows. These are large tunnels, four to eight inches in diameter. To each set of tunnels there are numerous entrances, some partly concealed in brush or beside logs or stumps, and some are in the open. Those in the open are less used as entrances than as places for receiving the loose earth which the animals excavate. A pile may contain nearly a cubic yard of earth and stones. Many of the burrows are shallow, and cave-ins are common. Breaks in the roof of a burrow are not repaired, although debris is removed from the burrow itself. The burrows seem not to be constructed according to a system, but are extended to take in whatever brush, logs or other cover is available. They are commonly dug through damp or muddy soil. Small streams flow through some burrows. Such partly flooded runs seem to be favored by the animals. The nest of a mountain beaver excavated by Scheffer (1929: 15), under the roots of a fallen tree, was oval in shape, twenty inches high and 13 inches wide. The nest was protected from flooding by a basal chamber, or basin, six inches beneath the nest. Two drainage tunnels lead away from this basal chamber. The nest was composed of the leaves and stems of bracken laced together with grass and fine twigs. Two other nests examined by Scheffer measured 17 by 18 inches and 19 by 17 inches. Both were about two feet beneath the surface. Around Puget Sound the mountain beavers mate in early March. The young number two to three, rarely four, per litter and are born in early April. Mountain beavers enter water readily but wade rather than swim. They are rather noisy, splashing in water and breaking twigs or rustling leaves on the ground. They climb bushes and saplings, clipping off branches for food as they ascend. According to Scheffer (1929: 15) they leave the stubs of branches attached to the trunk to facilitate their descent. Twice a mountain beaver was found several feet up in a sapling. In both saplings the animal had clipped the branches close to the trunk and was desperately reaching with its hind feet for missing branches. When the observer came near, one animal squealed, tumbled to the ground, and scurried frantically to its burrow.