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, Michoacán, 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.
Figs. 1-6. Skulls of the type specimens of two subspecies of Microtus mexicanus. XI.
Figs. 1-3. Microtus mexicanus fundatus.
Figs. 4-6. Microtus mexicanus salvus.
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.