Feb., 1912. Mammals of Illinois and Wisconsin — Cory. 183 



ern distribution than the Norway Rat. Since its introduction into 

 this country it has become well established and is not uncommon in 

 Louisiana and other Southern states. So far as known, it has not 

 been taken in Illinois. 



The three species may be distinguished by the following characters: 



Length from nose to root of tail generally greater than length of tail (never less); 

 upper parts brown; under parts brownish gray. A very common species. 



Brown Rat or Norway Rat. 



Epimys norvegicus. 



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



under parts not white or yellowish white. Doubtful if it occurs within our 



limits. Black Rat. 



Epimys rattus. 



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



under parts yellowish white or whitish. Not known to occur in Illinois and 



Wisconsin. Roof Rat. 



Epimys alexandrinus. 



Subfamily CRICETIN^E. 



This subfamily is of almost cosmopolitan distribution. The 

 tubercles on molars of upper jaw are arranged in two longitudinal 

 rows. 



Genus PEROMYSCUS Gloger. 



Peromyscus Gloger, Hand. u. Hilfsb. Naturg., I, 1841, p. 95. Type 

 Peromyscus arboreus Gloger = Mus sylvaticus nove- 

 boracensis Fischer. 



Molars or grinding teeth with two rows of tubercles 



Crowns of Upper - - . ^ ^ - 



Molars. on roundcd points or crowns; zygoma very slender; 



tail at least 2 inches long; belly white or whitish; soles of hind feet 

 with 5 or 6 tubercles or wart-like excrescences; size approaching that 

 of the House Mouse. 



Dental formula: I. ^— ^, C. ^^, Pm. ^^, M.^^=i6. 

 I - 1 0-0 0-0 2,-^ 



FIELD KEY TO OUR SPECIES. 



GROUP 1. Species which occur in about the northern two-thirds of Illinois and 

 about the southern half of Wisconsin. 

 Tail less than 2.75 inches long. Prairie White-footed Mouse. 



Peromyscus maniculatus bairdi, p. 190. 

 Tail more than 2.75 inches long. Northern White-footed Mouse. 



Peromyscus leucopus noveboracensis , p. 185. 



