Possibly intergradation occurs between _C. californicus occidentalis_ and _C. gapperi_. Nevertheless, I fail to find evidence of such intergradation. I have taken _C. gapperi nivarius_ and _C. californicus occidentalis_ within ten miles of each other, but each retained its distinctive characters with no evidence of intergradation. In the case of _C. gapperi saturatus_ and _C. californicus occidentalis_ the proof is less conclusive. In spite of numerous attempts to trap _Clethrionomys_ in the area geographically intermediate between their ranges, I have taken none. Though common along the coast, _occidentalis_ becomes progressively scarcer to the east, being rare in the vicinity of Seattle and apparently absent from the western base of the Cascades. So far as is known, the ranges of _caurinus_ and _occidentalis_ are separated by the Fraser River. MICROTUS PENNSYLVANICUS and MONTANUS.--The Pennsylvania meadow mouse is closely related to _Microtus montanus_. Certain races of _montanus_, notably those from southern Oregon, California, and northern Nevada, closely resemble _pennsylvanicus_ externally and cranially. From the central part of its range northward, _montanus_ becomes progressively less like _pennsylvanicus_. The races _nanus_ and _canicaudus_ are quite different from _pennsylvanicus_ both externally and cranially, and in addition the anterior loop of the second molar is less constricted; often it is not constricted at all. _Microtus pennsylvanicus_ and _Microtus montanus_ occur together over parts of the northern Rocky Mountains. Where the two species came together, _pennsylvanicus_ occurs with races of _montanus_ that are most unlike it. _Microtus pennsylvanicus kincaidi_ closely resembles races of _Microtus montanus_ that occur in southern Oregon, California, and Nevada. It is larger, darker, and longer-furred than _Microtus pennsylvanicus funebris_ from Washington. Seemingly _pennsylvanicus_ and _montanus_ diverged previous to Vashon-Wisconsin Time. The stock that gave rise to _montanus_ spread over the Great Basin while _pennsylvanicus_ ranged farther east. Some of the _montanus_ stock worked northward in the Rocky Mountains. _Microtus pennsylvanicus_ had meanwhile moved westward to the Rocky Mountains. The two stocks met and behaved as full species. _Microtus pennsylvanicus_ probably occupied northeastern Washington in the interglacial cycle preceding the Recent. The advance of the Wisconsin ice eliminated most of these mice. The glacier dammed the Columbia River and caused it to turn southward from its basalt-marginal course and take a path over the Columbian Plateau. Along this glacial river a population of _Microtus_ persisted to become _kincaidi_.