Frs., 1912. Mammatrs 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. Brack 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. Roor Rat. 
Epimys alexandrinus. 
Subfamily CRICETIN~. 
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 F 
Molars. on rounded 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. ES 0S = Pay 37 3_ 16, 
I-1 o-0 o-o0 3-3 
FIELD KEY TO OUR SPECIES. 
GROUP 1. Species which occur in about the northern two-thirds of Ilinois 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. 
