Some measurements not given above are included in Table 1, together with the number of specimens and the sex if restricted to one sex. So that the variability can be evaluated more adequately, the coefficient of variability and 2 times the standard error of the mean are included for the measurements in two series. The series consist of all the adult specimens (with a condylobasilar length of 24.0 mm. or more) of both sexes from the areas specified. Various barriers are shown in Fig. 2 for comparison with the distributions of the subspecies and the localities of known occurrence shown in Fig. 1. _Microtus montanus_ has not been taken in the Black Hills area of extreme northeastern Wyoming. Suitable montane habitat is present and both _Microtus pennsylvanicus insperatus_ and _Microtus longicaudus longicaudus_ occur there. The arid basin of the Powder River presumably is a barrier that has prevented _M. montanus_ from reaching this area. REFERENCES CITED BAILEY, V. 1900. Revision of American voles of the genus _Microtus_. N. Amer. Fauna, 17:1-88, June 6. 1917. A new subspecies of meadow mouse from Wyoming. Proc. Biol. Soc. Washington, 30:29-30, February 21. CARY, M. 1911. A biological survey of Colorado. N. Amer. Fauna, 33:1-256, August 17. 1917. Life zone investigations in Wyoming. N. Amer. Fauna, 42:1-95, October 3. DAVIS, W. B. 1939. The Recent mammals of Idaho. 400 p., front., illus., maps, diagrs., Caldwell, Id., The Caxton Printers Ltd., April 5. DURRANT, S. D. 1952. Mammals of Utah, taxonomy and distribution. Univ. Kans. Publ., Mus. Nat. Hist., 6:1-549, 91 figs, in text, 30 tables, August 10. ELTON, C. 1942. Voles, mice and lemmings; problems in population dynamics. 496 p., illus. (maps), tables, Oxford, The Clarendon Press, London. GOIN, O. B. 1943. A study of individual variation in _Microtus pennsylvanicus pennsylvanicus_. Jour. Mamm., 24:212-224, June 7. HALL, E. R. 1938. Notes on the meadow mice _Microtus montanus_ and _Microtus nanus_ with descriptions of a new subspecies from Colorado. Proc. Biol. Soc. Washington, 51:131-134, August 23. HILL, E. A. 1935. Cranial foramina in rodents. Jour. Mamm. 16(2):121-129. HOWELL, A. B. 1924. Individual and age variation in _Microtus montanus yosemite_. Jour. Agr. Res., 28(10):977-1015, June 7. KELLOGG, R. 1922. A study of the California forms of the _Microtus montanus_ group of meadow mice. Univ. California Publ. Zool., 21:245-274, April 18. MERRIAM, C. H. 1891. Results of a biological reconnaissance of south-central Idaho. N. Amer. Fauna, 5:1-113, July 30. PIPER, S. E. 1909. The Nevada mouse plague of 1907-8. U. S. Dept. Agr., Farmers' Bul. 352, pp. 1-23, 9 figs. WARREN, E. R.