NEOTOMA CINEREA.--The wood rat found over most of Washington. _Neotoma c. occidentalis_, probably entered the state from eastern Oregon early in the Recent and spread over most of the state. The wood rat of the Blue Mountains (_alticola_) probably developed from _occidentalis_. A coastal race of the wood rat (_fusca_) occurs in western Oregon. This race, if it occurred in western Washington in pre-Vashon Time, was eliminated in Vashon Time or subsequently. In Oregon it lives in deep forests (Bailey, 1936: 174). In Washington _occidentalis_ occupies but a small part of the ecologic niche occupied by _fusca_ in Oregon. Elimination of _fusca_ from Washington through competition with _occidentalis_ seems highly improbable. Should _fusca_ ever cross the Columbia River and become established it would probably spread to a considerable part of western Washington. SYNAPTOMYS BOREALIS.--The lemming mouse seems now to be retreating northward and was an arctic species forced southward by the Vashon-Wisconsin glaciers. Unlike other alpine species, it seems to be unable to exist for long in isolated mountain areas. PHENACOMYS INTERMEDIUS.--The heather vole, like the lemming mouse, probably was forced southward by the Vashon-Wisconsin glaciers. In the Cascade and Rocky mountains it found suitable habitat and spread southward to almost the lower end of the Sierra Nevada in California. The Cascade race moved eastward on the Puget Bridge to the Olympic Mountains after the retreat of the Vashon ice, and northward in the Cascades. Northeastern Washington was reinvaded by the subspecies of the Rocky Mountain Fauna. CLETHRIONOMYS GAPPERI and CALIFORNICUS.--The two species of red-backed mouse found in Washington were probably distinct in pre-Vashon Times. At the maximum extent of the Vashon ice, _californicus_ was probably found in western Oregon and _gapperi_ in the Blue Mountains, where _idahoensis_ was developed, and in the southern Cascades (_cascadensis_). During or shortly after Vashon Time, _gapperi_ crossed the Puget Bridge to become established in the Olympic Mountains. After the retreat of the ice, _gapperi_ moved northward and eastward from the Cascades and _californicus_ crossed to western Washington from Oregon and displaced _gapperi_ from the lowlands. Hinton (1926: 215) separated the American _Clethrionomys_ into three groups, suggesting that Old World counterparts of each group exist. Davis (1939: 308) gives an excellent analysis of the emigrational history of the American species, and points out the close relationship of the Siberian and Arctic-American forms. He suggests that the _rutilus_ group invaded Arctic-America from Siberia at the close of the last glaciation.