KEY TO LAND MAMMALS OF NORTHEASTERN NORTH AMERICA 93 



GENERA. OF MURIDAE 



Grinding teeth with tubercles arranged in three 

 transverse rows very distinct in teeth of upper 

 jaw (subfamily Mur in ae, old world rats and 

 mice) Mu8,p.94 



Grinding teeth with tubercles arranged in two 

 rows, or without distinct tubercles of any kind 

 Crowns of grinding teeth with tubercles ar- 

 ranged in two rows (subfamily C r i e e t i - 

 nae, American rats and mice) 

 Upper front teeth grooved (harvest mice) Reithrodontomys, p. 95 

 Upper front teeth not grooved 



Skull with a distinct ridge over eye- 

 socket; fur coarse; belly not pure 

 white; total length over 230 (9) (rice 



field mice) Oryzomys, p. 96 



Skull without ridge over eye-socket; 

 fur fine; belly pure white; total length 



under 215 (8i) (white-footed mice) .. Peromy8cu8,p. 96 



Crowns of grinding teeth divided into loops, 

 or triangles formed by plates of hard en- 

 amel inclosing a softer substance (dentine) 

 Upper front teeth narrow, compressed, 

 the antero-posterior diameter of each 

 much greater than the transverse di- 

 ameter; body slender, tail always 

 long ; eyes and ears large ; belly white 

 (subfamily Neotominae, wood 



rats and cave rats) Neotoma,p. 98 



Upper front teeth broad, the antero-pos- 

 terior diameter of each less than trans- 

 verse diameter ; body clumsy ; tail 

 usually short ; eyes and ears small ; 

 belly generally not white (subfamily 

 Microtinae, voles, lemmings, 

 muskrats etc.) 



Lower front teeth short, the roots 

 terminating on inner side of grind- 

 ing teeth (lemmings) 

 Upper front teeth grooved ; ears 

 small but well formed; color 

 not changing to white in win- 

 ter ; tail covered with short 



hairs Syuaptomys, p. 99 



Upper front teeth not grooved, 

 ears rudimentary; color white 

 in winter, tail with a brush of 

 stitf hairs nearly as long as 

 itself Dicrostonyx, p. 101 



