FEB., 1912. Mammats oF ILLInois AND WIsconsIN — Cory. 213 
Genus MICROTUS Schrank. 
Microtus Schrank, Fauna Boica, I, 1798, p.72. Type Microtus terrestris 
Schrank = Mus arvalis Pallas (by elimination). 
Form thick set; tail short; ears short; lower incisors extending in 
jaw back of molars; molars without pronged roots; crowns of molars 
with irregular triangles or loops; plantar tubercles (wart-like excre- 
scences on sole of foot) 4 to 6. 
2-8, Pm. —; M 373 16. 
Dental formula: I. =. : 
I=1 ° o-0 3-3 
Three recognized subgenera are represented within our limits, 
which may be characterized as follows in our species: 
A. Plantar tubercles 6. 
Crown of third upper molar with five or 
more irregular loops, the middle one’ 
forming three closed triangles; mamme 8 
in our species, 4 pectoral and 4 inguinal. 
Subgenus MICROTUS, p. 214. 
Crowns of upper molars and sole of foot. 
B. Plantar tubercles 5. 
Crown of third upper molar with 4 irregular 
loops, the middle ones forming two closed 
triangles; mamme 6, 4 inguinal, 2 pec- 
toral; skull narrow and high. 
Subgenus PEDOMYS, p.218. 
Crown of third upper molar with 4 irregular 
loops, the middle ones forming two closed 
triangles; mamme 4, inguinal; skull flat 
3 and wide. Subgenus PITYMYS, p. 222. 
FIELD KEY TO OUR SPECIES. 
A. Total length more than 5.50 inches; tail more than 1.10 inches long. 
Upper parts more or less grizzly brown; under parts buffy gray or pale brownish 
gray; tail usually Jess than 1.60 inches long; plantar tubercles 5; mamme- 
6. Prairie Meapow Mouse or VOLE. 
Microtus ochrogaster, p. 218. 
Upper parts brown, but lacking the ‘‘grizzly’’ appearence of M. ochrogaster; 
under parts slaty plumbeous, often with a slight wash of cinnamon; tail 
usually more than 1.60 inches long; plantar tubercles 6; mamme 8 (in 
our species). Meapow Mouse or VOLE. 
Microtus pennsylvanicus, p. 214. 
