SUBFAMILY MURINJE 147 



Subfamily MURIWE. 



Genus Mus Linnasus. 



Three molars in upper jaw, the tubercles on the crowns arranged 

 in three longitudinal rows ; crown of anterior molar larger than 

 both the others combined; third molar very small; fore feet with 

 four developed toes and a rudimentary thumb with a short nail; 

 five developed toes on hind feet; ears large, prominent; nose 

 acute ; tail long, usually nearly naked and scaly. 



Dental formula I. 1/1 ; M. 3/3=16. 



A single form occurs in Iowa and the United States. 



House Mouse. 

 Mus musculus. p. 52 

 Genus Rattus G. Fisher. 



Premolars wanting; upper molars with tubercles on. crowns ar- 

 ranged in three longitudinal rows; "molars slightly graduated, in 

 size from first to third, the anterior tooth not tending to assume 

 the main function of the tooth row, the posterior tooth not tending 

 to disappear, enamel folding of upper molars directly referable 

 to a simple 9-cusped pattern and its reductions, the outer margins 

 of M 1 and M 2 never with more than 3 cusps, the inner margin 

 of same teeth never with more than 2 cusps, M 1 usually with 5 

 roots, its first lamina not distorted by the backward displacement 

 of the antero-internal tubercle ; upper incisor moderately com- 

 pressed, set at such an angle that its outer side is worn smoothly 

 away by action of lower teeth. ' ' 37 



Dental} formula^-I. 1/1 ; M. 3/3=16. 

 Key to Species of Rattus. 



Ears moderate; length of tail usually less than length of head 

 and body; color grayish brown ?bove, white below; hind foot, 38 

 to 46 mm. 



Brown Rat. 

 Rattus norvegicus. p. 54 



Ears larger, broader; tail longer than head and body; entire 

 pelage dusky in color; hind foot, 33 to 37 mm. (Probably not now 

 occurring within our limits.) 



Black Rat. 

 Rattus rattus. p. 68 



"Miller, G. S., Proc. Biol. Soc. W&sh., XXIII, 58, 1910. 



