MAMMALS 291 



1. Mus L. With the characters of the subfamily; tail scantily 

 haired; size small: 4 semidomesticated species in the United States. 



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

 under parts ashen; length 170 mm.; tail 85 mm.; hind foot 17 mm.: 

 everywhere in houses; often in fields and woods; about 5 litters annually 

 of 4 to 9 each. 



2. Rattus Fischer (Mw5 L.). With the characters of the subfamily; 

 tail scaly; size large: 3 American species. 



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

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

 tail 200 mm.; hind foot 43 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. 



R. rattus (L.). Black rat. Body slenderer, 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. 



R. alexandrinns (Geoffroy). Roof-rat. Body brown above, white 

 beneath; length 425 mm.; tail 237 mm.; hind foot 40 mm.: southern 

 United States; common in some 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; and very many subspecies. 



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. Reilhrodontomys. 



b2 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). 



di Belly pure white; fur fine 3. Peromyscus. 



do Belly not pure white; fur coarse 4. Oryzomys. 



a 2 Tubercles of molars form flattened S-shaped loops 5. Sigmodon. 



I. Onychomys Baird. Grasshopper mice; scorpion mice. Body 

 stout; fur dense and velvety; tail short; fore feet strong; ears hairy: 

 2 species, in western America; nocturnal burrowing animals which 

 feed largely on insects, scorpions and other small animals; several litters 

 of from 2 to 6 young each raised each year; the animals do not hibernate. 



