FEB., 1912. MAMMALS OF ILLINOIS AND WISCONSIN CORY. 173 



SECTION 2. Total length less than 12 inches. 



PART 1. Front of upper incisors (front teeth) not grooved. 



Grinding teeth without pronged roots in adults; back and sides of body 

 nearly uniform in color. Genus MICROTUS, p. 213. 



Grinding teeth with pronged roots in adults; back with chestnut dorsal 

 stripe (middle of back) in decided contrast with gray-brown of 

 sides. Genus EVOTOMYS, p. 208. 



PART 2. Front of upper incisors (front teeth) with distinct groove 

 near outer edge. Genus SYNAPTOMYS, p.233. 



KEY TO THE SPECIES. 



This key is largely based on external characters for use in the field and is intended 

 to apply to adult specimens only. 



GROUP 1. Total length (including tail) more than 10 inches. 



SECTION 1. Total length more than. 1 8 inches. General color brown ; fur thick 

 and fluffy; tail hairless for the greater part of its length and laterally 

 compressed. MUSKRAT. Fiber zibethicus, p. 225. 



SECTION 2. Total length less than 18, but more than 10 inches. 

 PART 1. Tail with little or no hair; crowns of molars with tubercles. 



Length from nose to root of tail less than length of tail; general color 

 blackish; under parts not white or yellowish white. Claimed to 

 have occurred within our limits but no definite records. 



BLACK RAT. Epimys ratlus, p. 181. 



Length from nose to root of tail less than length of tail; upper parts 

 brownish; under parts yellowish white or whitish. This species 

 has not been observed within our limits but may 

 occur. ROOF RAT. Epimys alexandrinus, p. 181. 



Length from nose to root of tail greater than length of tail 

 (never less) ; upper parts brownish; under parts brown- 



Epimys. ish gray. Common in houses and barns. 



Crowns of Upper Molars. . T _, _ TT 



(Enlarged.) NORWAY RAT. BROWN RAT OR HOUSE RAT. 



Epimys norvegicus, p. 180. 



PART 2. Tail covered with hair, crowns of molars with irregular triangles or 

 loops. Upper parts brownish; under parts whitish; feet 

 white; ears large; occurs only in southern Illinois. About 

 the size of large House Rat, but readily distinguished from it 



Crowns cf Upper Molars. by its hair - covered tail . lar g e ears . and white feet - 



(Enlarged.) ILLINOIS WOOD RAT. Neotoma floridana illinoensis, p. 204. 



GROUP 2. Total length less than 10 inches; tail more than 2 inches long. 



A. Species which occur in southern Illinois. 

 SECTION 1. Tail more than 2 inches, but less than 3.75 inches long. 



PART 1. Under parts white or grayish white; upper parts brown, but not 



golden brown ; middle of back darker brown that the sides. 

 Total length more than 7 in. (180 mm.); tail vertebra? about 3.12 in. (80 

 mm.) or more. WESTERN COTTON MOUSE. 



Peromyscus gossypinus megacephalus, p. 196. 



