Average and extreme measurements of six adult males from near Pocatello, Bannock County, Idaho, and nine adult males from near Afton, Lincoln County, Wyoming, are, respectively, as follows: total length, 143(135-150), 163(143-179); length of tail, 35.1(33-38), 42.8(36-49); caudal index, 32.0(28.0-33.1), 35.7(30.6-41.9); hind foot, 18.9(18-20), 18.8(17-20); condylobasilar length of skull, 24.4(24.0-26.0), 25.6(24.5-26.2); alveolobasilar length, 14.1(13.7-14.5), 14.6(13.8-15.0); palatilar length, 13.2(12.9-13.6), 13.8(13.2-14.5); alveolar length of upper molar tooth-row, 6.3(6.1-6.5), 6.3(6.0-6.6); depth of braincase, 7.7(7.5-7.9), 8.0(7.7-8.3); lambdoidal breadth, 11.4(11.0-11.7), 12.0(11.3-12.7); prelambdoidal breadth, 9.1(8.6-9.4), 8.7(8.0-9.4); zygomatic breadth, 14.3(13.8-14.7), 15.3(14.4-16.3); interorbital breadth, 3.6(3.5-3.7), 3.5(3.3-3.7). The average length of the nasal bones in the series from Pocatello is 7.1 mm. The averages, which have not been included in Table 1, for three measurements of the series from Carbon County, Wyoming, are as follows (Encampment, males; Encampment, females; Savery, males; and Savery, females, respectively): alveolobasilar length, 14.4, 14.3, 14.5, 14.3; interorbital breadth, 3.5, 3.4, 3.5, 3.4; depth of braincase, 7.8, 7.6, 7.9, 7.6. Additional measurements are included in Table 1 for other series.
_Discussion._--The name _Microtus montanus caryi_ Bailey is here placed in synonymy under _M. m. nanus_ (Merriam). Vernon Bailey (1917) in his description of _caryi_ made four assumptions that have been found to be entirely or partly invalid. First, he assumed that this is an "extreme variant which gradually changes in characters across Nevada and Utah, and reaches its maximum variation in Wyoming." The differences pointed out in subsequent descriptions of subspecies found in the above area do not show a gradual change in any character, or in the number of characters, nor is _caryi_ an extreme when compared with the other subspecies. Second, _Microtus nanus_ was not, as Bailey assumed, a different species than _Microtus montanus_. Third, he assumed that the characteristics of adults of _nanus_ were adequately ascertainable from the thirteen topotypes available to him. Subsequent sampling from Idaho shows that the series of specimens available to Bailey was made up mostly of young and subadult animals. Finally, _caryi_ does not occupy as Bailey stated "the meadows along streams in the arid sagebrush country of the Bear River, Green River, and Wind River valleys" exclusively, or characteristically. When the localities from which the species actually is known are plotted, it seems that the arid basin serves as a barrier and that the species is more commonly and abundantly found in montane meadows in the Transition and Canadian life-zones.