From _M. c. salvus_, the subspecies next to the westward, _fundatus_ differs as follows: Averaging larger in all parts measured; less reddish on upper parts; underparts with more reddish color but the reddish of lighter tint; perineal region buffy instead of plumbeous; nasals with premaxillary borders laterally concave rather than straight; superior outline of skull with nasal segment sloping anteroventrally and interparietal segment sloping posteroventrally rather than straight; posterior margin of orbit inclined posterolaterally thus forming an acute angle, instead of a right angle, with sagittal plane of braincase; zygomatic arches parallel rather than bowed outward; incisive foramina narrower; tympanic bullae more inflated in vertical plane. Relying on Bailey's (N. Amer. Fauna, 17:55, 1900) description of _Microtus fulviventer_, _fundatus_ differs in much larger tympanic bullae. _Remarks._--The series of 59 specimens includes individuals of several ages of both sexes. This has been a great advantage in making comparisons with individuals of geographically adjoining subspecies since individuals of the same age and sex could be compared. When the skulls are laid top-side down on a flat surface the occiput is much higher than in _salvus_. Our specimens, taken in the dry season, were trapped mostly in runways beneath a dense growth of grass underneath a rail fence. _Specimens examined._--Total, 59, distributed, with respect to the town of Patzcuaro, as follows: 3-1/2 mi. S, 7,900 ft., 9; 4 mi. S, 7,800 ft., 16; 5 mi. S, 7,800 ft., 26; 9 mi. SE, 8,000 ft., 8. [Illustration: 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._] #Microtus mexicanus salvus# new subspecies _Type._--Female, adult, skin and skull; No. 52099, Chicago Natural History Museum; Mount Tancitaro, 11,400 ft., Michoacan, Mexico; July 19, 1941; obtained by F. C. Wonder, original No. 1163. _Range._--Known only from Mount Tancitaro at elevations of 7,800 to 11,400 feet. _Diagnosis._--Size medium (see measurements); color brown overlain with cinnamon; premaxillary borders of nasals straight; superior outline of skull nearly straight; posterior margin of orbit at right angle with long axis of skull; zygomatic arches bowed outward; incisive foramina wide; tympanic bullae small.