SUBFAMILY MURINE 147 



Subfamily MURIN/E. 



Genus HIks Liniueus. 



Three molars in upper jaw, the tubercles ou the crowns arran2;ed 

 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 witJi a short nail; 

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

 acute; tail long, usually nearly naked and scaly. 



Dental form ula^l. 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 assiime 

 the main fimction of the tooth rovf, the posterior tooth not tending 

 to disappear, enamel folding of upper molars directly referable 

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

 of M^ and M- never with more than 3 cusps, the inner margin 

 of same teeth never wdth more than 2 cusps, M^ 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. "^^ 



Dental formula^l. V\- M. 3/3=16. 



Key to Species of Rattus. 



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



and body; .color grayish bro\ra above, 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 



"Mmer, G. S., Froc. Biol. Soc. Wash., XXIII, 5S, 1910. 



