FEB., 1912. MAMMALS oF ILLINOIS AND WisconsIN— Cory. 171 
Family MURIDA. Rats and Mice. 
The Muride is the largest known family of mammals. Its members 
are practically cosmopolitan and number at least 450 known species, 
representing nearly 100 genera. They are all animals of small size, 
the largest being the Muskrat (Fiber). They have no premolars and 
never more than three cheek teeth (molars), the differences in the latter 
being often generically diagnostic. The antorbital foramen is large, 
the edges forming a nearly perpendicular slit in front of the zygoma 
in most species; * molars tuberculate or with enamel folds and with or 
without roots; the tibia and fibula are anchylosed below, and post- 
orbital processes are wanting. Four subfamilies are represented in 
North America including the introduced subfamily Murine containing 
Old World Rats and Mice. 
KEY TO THE GENERA 
WHICH OCCUR WITHIN OUR LIMITS. 
GROUP 1. Grinding teeth (molars) of upper jaw with tubercles or rounded points 
on crowns extending in three longitudinal rows; tail nearly 
REGD naked. 
Total length (including tail) less than 10 inches. 
Mus. Genus MUS, p. 176. 
ee Ah al Total length (including tail) more than 10 inches. 
: Genus EPIMYS, p. 189, 
GROUP 2. Grinding teeth (molars) of upper jaw with tubercles or rounded points 
on crowns extending in éwo longitudinal rows. 
SECTION 1. Skull showing a ridge over the eye socket. 
Total length over 8.75 inches; hind feet decidedly large; 
Peromyscus. incisors not grooved. Genus ORYZOMYS, p. 202. 
Crowns of Molars. : 
(Enlarged.) 
SECTION 2. Skull not showing distinct ridge over eye socket. 
Total length less than 8.75 inches; hind feet not decidedly large; incisors 
not grooved. Genus PEROMYSCUS, p.183. 
Size small, total length less than 5.50 inches; front of upper incisors with 
longitudinal groove. Genus REITHRODONTOMYS, p. 201. 
GROUP 3. Crowns of grinding teeth (molars) with loops or irregular triangles 
clearly shown in upper jaw. 
CAG Ly Cw wn ) SECTION 1. Total length more than 12 inches. 
Ny ele 
NG NY CN Tail covered with hair; ears large; belly whitish. 
Genus NEOTOMA, p. 204. 
Ni Sars 
Sear. al weeps Tail laterally compressed and naked for greater part of 
(Balnieeds) its length. Genus FIBER, p. 225. 
*See fig. 5, p. 96. 
