MEASUREMENTS OF BREADTH. Zygomatic breadth.--greatest transverse width. Interorbital breadth.--the breadth of the interorbital constriction. Lambdoidal breadth.--between the lateralmost points of the lambdoidal crest. Prelambdoidal breadth.--between the medialmost margins of the prominent fenestrae in the posterodorsal parts of the squamosal bones. To these fenestrae Howell (1924:995) applied the adjective "prelambdoidal," but other authors have used other names (see Hill, 1935:127). Depth of braincase.--shortest distance from the ventral side of the cranium at the suture between the basioccipital and basisphenoid bones to the dorsal surface of the cranium (usually not perpendicular to the long axis of the skull). The history of our knowledge of _Microtus montanus_ in this area begins with the early work of the United States Bureau of Biological Survey directed by C. H. Merriam (1891), and participated in by Vernon Bailey (1900, 1917), Merritt Cary (1911, 1917), and others. The changes in nomenclature which grew out of increased understanding of these mice through additional collecting are expressed in the synonymies under the accounts of subspecies. As a result of my studies two of the three subspecific names previously proposed for mice from this area have been retained although changes are proposed in the ranges assigned to the two subspecies and two additional heretofore unrecognized subspecies are named and described. Furthermore the additional specimens and my studies of variation make modifications in the characterization of these subspecies necessary. Not all of the samples here assigned to a single subspecies are identical and I therefore list and discuss some of the local variants. Numerous members of summer field parties from the Museum of Natural History at the University of Kansas collected most of the specimens studied and wrote field notes that have been helpful. I am grateful to these persons and to Professor E. R. Hall and Dr. R. H. Baker for their assistance and helpful suggestions. Specimens in the following museums were made available by their respective curators: Chicago Natural History Museum by Mr. Colin C. Sandborn, The Museum of Zoology at the University of Michigan by Dr. E. T. Hooper, The American Museum of Natural History by Mr. G. G. Goodwin, The United States National Museum by Dr. David H. Johnson and the Biological Surveys Collection by Miss Viola S. Schantz. A fellowship from the National Science Foundation made possible the studies at the museums other than at the University of Kansas. =Microtus montanus nanus= (Merriam) _Arvicola (Mynomes) nanus_ Merriam, N. Amer. Fauna, 5:62, July 30, 1891. _Microtus montanus nanus_, Hall, Proc. Biol. Soc. Washington, 51:131, August 23, 1938.