Four habitats may be distinguished in the Humid subdivision and further subdivision is possible. The dominant and most extensive habitat is the forest. Dominant mammalian species include: _Peromyscus maniculatus_, _Sorex trowbridgii_, _Sorex obscurus_, _Tamiasciurus douglasii_, _Clethrionomys californicus_, _Aplodontia rufa_, _Glaucomys sabrinus_ and _Odocoileus hemionus_. Mammals are scarce and nocturnal forms prevail. As a rule, a line of mouse traps set in a forest habitat will take principally _Peromyscus maniculatus_ with a few _Sorex trowbridgii_ and _Sorex obscurus_ and rarely a _Clethrionomys californicus_. In some places, especially where the moss is deep, a line of mouse traps will catch only shrews. [Illustration: FIG. 13. Humid subdivision of the Transition Life-zone, Headley's Marsh, five miles east of Granite Falls, Washington, June 4, 1938. (Fish and Wildlife Service photo by Victor B. Scheffer, No. 60.)] A second important habitat is the deciduous jungle. This differs from the forest habitat in that the dominant trees are of the deciduous type and in that understory vegetation, such as shrubs and annuals, is dense. The jungle habitat occurs in ravines and in valleys of streams and rivers and, in general, covers the lower, poorly drained portions of the Humid subdivision of the Transition Life-zone. Mammals are abundant and varied in the jungle habitat. The deer mouse (_Peromyscus maniculatus_) is the most common mammal but a line of mouse traps might also catch: _Neurotrichus gibbsii_, _Scapanus orarius_, _Sorex vagrans_, _Microtus oregoni_, or _Zapus p. trinotatus_. The mountain beaver, snowshoe rabbit, and Townsend chipmunk also occur there. The prairies form a third habitat. These areas of native grasslands are of minor extent but are the principal home of several races of gophers and the Townsend mole (_Scapanus townsendii_). Deer and elk also browse on the prairies. [Illustration: FIG. 14. Timbered arid subdivision of the Transition Life-zone, Kettle Falls on the Columbia River (now beneath Coulee Dam backwater), Stevens County, Washington, June 15, 1938. (Fish and Wildlife Service photo by Victor B. Scheffer, No. 72.)] A fourth habitat is the marsh. In it there is here included the sphagnum bogs and marshy shores of lakes and streams. This habitat is characterized by damp ground, standing water, and dense vegetation. Typical mammalian species include the water shrew (_Sorex bendirii_), Townsend meadow mouse, muskrat and mink. Other habitats, such as aerial for the bats and aquatic for the beaver and otter, might be listed. THE ARID TIMBERED SUBDIVISION OF THE TRANSITION LIFE-ZONE In Washington the Arid timbered subdivision of the Transition Life-zone is the open, pine forest. Because of the aridity of this habitat, marshes and streamside thickets are uncommon, but where habitats of this kind do occur they have a fauna distinct from that of other habitats.