_Comparisons._--From topotypes of its nearest relative, _Microtus mexicanus phaeus_ Merriam, _salvus_ differs as follows: Tail shorter; pelage with slightly more buffy (reddish) pigment which, nevertheless, is of a lighter tint; premaxillary borders of nasals straight rather than concave; superior outline of skull more nearly straight (less convex); inferior border of rostrum more nearly straight (less concave distally); palatal fossae uniformly shallower. Comparison with _M. m. fundatus_ is made in the account of that subspecies. _Remarks._--The degree of difference between _M. m. salvus_ and _M. m. fundatus_ exceeds that between _M. m. salvus_ and _M. m. phaeus_ or that between _M. m. fundatus_ and _M. m. mexicanus_. _Specimens examined._--Total, 14, from Mount Tancitaro, Michoacan, distributed, according to elevation above sea level, as follows: 11,400 feet, 8; 11,000 ft., 2; 7,800 ft., 1; no elevation recorded, 8. _Measurements of the two subspecies._--Average and extreme measurements of ten adult males of _M. m. fundatus_ and eight adult males of _M. m. salvus_, are, respectively, as follows: Total length, 147 (135-158), 141 (134-146); length of tail, 33.4 (30-39), 29.6 (25-32); length of hind foot, 21.1 (20-22), 20.0 (18-21); condylobasal length, 27.7 (27.0-29.0), 25.9 (25.5-26.2); occipitonasal length, 27.5 (26.8-28.5), 25.7 (25.3-26.0); length of nasals, 8.0 (7.7-8.4), 7.5 (6.9-8.1); zygomatic breadth, 16.4 (15.9-17.2), 15.0 (14.6-15.3); interorbital breadth, 3.7 (3.5-3.9), 3.5 (3.3-3.6); mastoid breadth, 12.7 (12.2-13.2), 11.7 (11.7-11.7); alveolar length of upper molar series, 7.0 (6.8-7.3), 6.7 (6.5-6.9); width of rostrum, 5.7 (5.1-5.9), 5.3 (5.3-5.3); palatilar length, 13.3 (12.7-14.3), 12.6 (11.9-12.9). Of _M. m. salvus_ only two specimens yield data for the first, second, sixth and eighth cranial measurements named above. For the loan of comparative material I am grateful to Dr. Hartley H. T. Jackson and Mr. Stanley P. Young of the Biological Surveys Collection in the United States National Museum, Messrs. Karl P. Schmidt and Collin C. Sanborn of the Chicago Natural History Museum, and for assistance with the field work to the John Simon Guggenheim Memorial Foundation and to Miss Annie M. Alexander. _Transmitted June 1, 1948._ 22-3717