FEB., 1912. MAMMALS OF ILLINOIS AND WISCONSIN CORY. 213 



Genus MICROTUS Schrank. 



Microtus Schrank, Fauna Boica, I, 1798, p. 72. Type Microtus lerrestris 



Schrank = M us 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. 



Dental formula: I. > C. > Pm. 



i-i 



o-o 



o-o 



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 ones 

 forming three closed triangles; mammae 8 

 in our species, 4 pectoral and 4 inguinal. 



Subgenus MICROTUS, p. 214. 



Crowns of upper molars and sole of foot. 



Plantar tubercles 5. 



Crown of third upper molar with 4 irregular 

 loops, the middle ones forming two closed 

 triangles; mammae 6, 4 inguinal, 2 pec- 

 toral; skull narrow and high. 



Subgenus PEDOMYS, p.2i8. 



Crown of third upper molar with 4 irregular 

 loops, the middle ones forming two closed 

 triangles; mammae 4, inguinal; skull flat 

 and wide. Subgenus PITYMYS, p. 222. 



FIELD KEY TO OUR SPECIES. 



A. Total length more than 5.50 inches; tail more than l.io inches long. 



Upper parts more or less grizzly brown; under parts buffy gray or pale brownish 

 gray; tail usually less than 1.60 inches long; plantar tubercles 5; mammae 

 6. PRAIRIE MEADOW 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; mammae 8 (in 

 our species). MEADOW MOUSE or VOLE. 



Microtus pennsylvanicus, p. 214. 



