FEB., 1912. Mammats oF ILLINoIs AND WIScoNnsIN — 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 
Vv sides. Genus EVOTOMYS, p. 208. 
Part 2. Front of upper incisors (front teeth) with distinct groove 
near outer edge. Genus SYNAPTOMYS, p.233. 
Synaptomys. 
Upper Incisors. 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 18 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 Io inches. 
Part 1. Tail with little or no hair; crowns of molars with tubercles. 
Length from nose to root of tail Jess 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. 
Biack Rat. Epimys rattus, p. 181. 
Length from nose to root of tail Jess than length of tail; upper parts 
brownish; under parts yellowish white or whitish. This species 
has not been observed within our lmits but may 
a = occur. Roor Rat. Epimys alexandrinus, p. 181. 
He forse Length from nose to root of tail greater than length of tail 
; (never less); upper parts brownish; under parts brown- 
ee a a ish gray. Common in houses and barns. 
i) er fe) . 
(Enlarged) foe 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 
by its hair-covered tail, large ears, and white feet. 
Crowns of Upper Molars. i ate 5 
(Enlarged.) ILL1no1s Woop 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 
WESTERN Cotton MOUSE. 
mm.) or more. 
Peromyscus gossypinus megacephalus, p. 196. 
