MAMMALS 299 



Key to the Subfamilies of Muridae 



El Molars with a more or less tuberculate biting surface (Fig. 160). 



bi Tubercles arranged in 3 longitudinal rows i. MurincB. 



bo Tubercles arranged in 2 rows (Fig. 160) 2. Cricetince. 



a2 Molars with a flattened surface and not tuberculate (Fig. 167). 



bi Tail long; body slender 3. Neotomince. 



b2 Tan short; body stout 4. Microtina. 



Subfamily i. Murinae. — Old world rats and mice. Form slender; 

 tail long and scaly and scantily haired; thumb rudimentary: 130 species, 

 in all parts of the old world except Madagascar; a few species have 

 followed man to the new world. 



Mus L. With the characters of the subfamily: 4 semidomesticated 

 species in the United States. 



M. musculus L. House-mouse. Body brownish gray in color; 

 length 170 mm.; tail 85 mm.; hind foot 17 mm.: everywhere in houses; 

 often in fields and woods; about slitters annually of 4 to 9 each. 



M. norvegicus Erxleben. {M. decumanus Pallas.) Common rat; 

 brown rat. Body brown above and gray beneath; length 400 mm.; 

 tail 180 mm.; hind foot 45 mm.: very common. This rat made its 

 appearance in Europe in eastern Russia in 1727, and rapidly spread 

 westward, driving out and replacing the black rat, which up to that 

 time had been the common house-rat. It appeared in America about 

 the time of the Revolutionary War. 



M . rattus L. Black rat. Body glossy black in color; length 400 

 mm.; tail 215 mm.; hind foot 37 mm.: during colonial times the common 

 house-rat in America, but now almost exterminated by the larger, 

 fiercer brown rat. 



M. alexandrinus Geoffroy. Roof-rat. Body brown above, white 

 beneath; length 390 mm.; tail 200 mm.; hind foot 35 mm.: southern 

 United States; common in sorhe places. 



Subfamily 2. Cricetinae.— American rats and mice. Body slender; 

 ears and eyes large; legs and tail long; tail well haired; upper molars 

 usually with tubercles arranged in 2 longitudinal rows (Fig. 160): 

 about 50 species in the United States. 



Key to the Genera of Cricetinae 



ai Tubercles on surface of upper molars in 2 distinct longi- 

 tudinal rows (Fig. 160). 



bi Upper incisors longitudinally grooved (Fig. 161) 2. Reithrodontomys. 



bz Upper incisors not thus grooved. 



Ci Soles of feet with 4 plantar tubercles i. Onychomys. 



C2 Soles of feet with 5 or 6 plantar tubercles (Fig. 163). 



