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 Pátzcuaro, 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., Michoacán, México; 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.
_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_.