FEB., 1912. MAMMALS OF ILLINOIS AND WISCONSIN CORY. 171 



Family MURID^. Rats and Mice. 



The Muridce 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 genetically 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 Murince 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 

 naked. 



Total length (including tail) less than 10 inches. 

 Mus Genus MUS, p. 176. 



Crowns of Molars. Total length (including tail) more than 10 inches. 



GenusEPIMYS lP .i8o. 

 GROUP 2. Grinding teeth (molars) of upper jaw with tubercles or rounded points 



on crowns extending in two longitudinal rows. 

 SECTION 1. Skull showing a ridge over the eye socket. 



Total length over 8.75 inches; hind feet decidedly large; 



Ptromyscus. 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.i83. 



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. 

 SECTION 1. Total length more than 12 inches. 



Tail covered with hair; ears large; belly whitish. 



Neotoma Genus NEOTOMA, p. 204. 



Crowns of Molars. Tail laterally compressed and naked for greater part of 



its length. Genus FIBER, p. 225. 



*See fig. 5, p. 96. 



