KEY TO LAND MAMMALS OF NORTHEASTERN NORTH AMERICA 93 



GENEKA OF MURIDAB 



Grinding teeth with tubercles arranged in three 

 transverse rows very distinct in teeth of upper 

 jaw (subfamily Murinae, 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 iu two rows (subfamily Crieeti- 

 nae, American rats and mice) 

 Upper front teeth grooved (harvest mice) Eeithrodontomys, 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 flue; belly pure white; total length 



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



• Crowns of grinding teeth divided into loops, 

 or triangles formed by plates of hard en- 

 amel inclosing a softer substance (dentine) 

 Upper frout 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 

 Mierotinae, voles, lemmings, 

 ^nnskrats etc.) 



Lower front teeth short, the roots 

 terminating on inner side of grind- 

 ing teeth (lemmings) 

 Upper front teetb grooved; ears 

 small but well formed; color 

 not changing to white m win- 

 ter ; tail covered with short 



hairs Synaptomys, p. 99 



Upper front teeth not grooved, 

 ears rudimentary; color white 

 in winter, tail with a brush of 

 stiff hairs nearly as long as 

 itself Dicrostonyx, p. 101 



