156 Order RODENTIA 



Bailey occurred over most of the state, and C. c. carolinensis 

 Rhoads inhabited the extreme southern part. By the middle 

 1800's beavers had been reduced to the point of being rare in the 

 state, and during the, late 1800's or early 1900's the native popu- 

 lation was exterminated. In 1935, beavers brought from Wis- 

 consin were released in Illinois; subsequently there were inva- 

 sions of beavers from Iowa and Indiana. These animals in- 

 creased in numbers and extended their range until by 1954 

 beavers were present in nearly half the counties of Illinois; 

 through transplantations or natural movements there were 

 nearly 600 colonies. All members of the species occurring in 

 Illinois at present probably belong to the subspecies C. c. michi- 

 ganensis. The natural range of the species occupies most of 

 Alaska, Canada, and the United States except parts of the 

 southeastern and Gulf Coast states and the arid parts of the 

 southwestern states. 



REITHRODONTOMYS MEGALOTIS (Baird) 

 Western Harvest Mouse 



Description. — The western harvest mouse, fig. 4, smaller 

 than the white-footed mouse, is similar to the house mouse except 

 in color. Its back and sides are brown, and its under parts are 

 silvery gray; the under side of its tail is whitish, and the upper 

 sides of its hind feet are white. Each upper front tooth (incisor) 

 has a groove down its face. The only other rodents now reported 

 in Illinois with such grooves are the plains pocket gopher, the 

 meadow jumping mouse, and the bog lemming. 



Length measurements: head and body 2]/z~3 inches (66-77 

 mm.) ; tail 2]4~2% inches (57-68 mm.) ; over-all 4^-5^ 

 inches (123-145 mm.); hind foot about Y% inch (16-18 mm.); 

 ear from notch about \/> inch (12-14 mm.). 



The skull averages 20.3 mm. (about >)4 inch) in length; in 

 width across the zygomatic arches it averages 10.4 mm. (about 

 y 8 inch). Dental formula: I 1/1, C 0/0, Pm 0/0, M 3/3. 



Life History. — Western harvest mice taken in Illinois 

 near Mount Carroll were found living among brome-grass, 

 goldenrod, blackberry, ragweed, and bluegrass. The vegetation 

 was tall but not thick. Harvest mice were quite rare in this 

 habitat and were associated with masked shrews, short-tailed 

 shrews, white-footed mice, and bog lemmings. 



