79. Ante-orbital opening long oval; four upper cheek teeth; Canada and 



northern half of U. S. 



Zapus Jumping Mouse 

 Ante 'Orbital opening narrowing to a slit below; three upper cheek teeth; 

 southern half of U. S. 



Reithrodontomys Harvest Mouse 



80. With a thread-like ridge along upper rim of eye-socket 81. 

 No such ridge 83. 



81. Skull rarely as much as 1% inches long; cheek teeth with two rows of 



cusps; large opening in palate near last upper cheek tooth, the latter 

 showing one posterior and two or more anterior planes 

 Oryzomys Rice Rat 

 Skull usually from 1% to 1% inches long; cheek teeth with three ir' 

 regular rows of cusps; none or a very small opening in palate near last 

 upper cheek tooth, this tooth usually having one large surface with 

 one smaller inner and slightly anterior one 82. 



82. Cheek tooth row almost as long as the space between incisor and begin' 



ning of cheek tooth row; skull usually about IV2 inches long 

 Rattus rattus Black Rat, Roof Rat 

 Cheek tooth row about one-half as long as the space between it and the 

 incisor; skull usually about P/j inches long 



Rattus norvegicus Barn, Gray or Norway Rat 



83. Cheek teeth with tubercles in three irregular rows; skull about % inch 



long and very delicate 



Mus musculus House Mouse 

 Cheek teeth tubercles in two rows; skull 1 to IV2 inches long; thin but 

 not particularly delicate 84. 



84. Last upper cheek tooth about one-half the size of the one before it; upper 



edge of orbit rounded; coronoid process of lower jaw (dorsal projec- 

 tion anterior to hinge) about as long as its width at base 

 Peromyscus White-footed or Deer Mouse 

 Last cheek tooth less than one-half the size of its neighbor; upper rim 

 of orbit sharp-edged; coronoid process about twice as long as its width 

 at base; Dakotas and Neb. west and south 

 Onychomys Grasshopper Mouse 



85. Skull about 2V2 inches long, almost as wide as long; five upper cheek 



teeth on each side, each, after the first, with a definite projection on 

 the outer side 



Aplodontia Mountain Beaver 

 Skull smaller; not more than four upper cheek teeth on each side, the 

 first sometimes 8-shaped and easily mistaken for two 86. 



86. Three upper cheek teeth on each side, each sometimes with several lobes 



and so appearing to be more 87. 



Four upper cheek teeth, the first sometimes very small 95. 



87. Upper incisors grooved on anterior surface 88. 

 Upper incisors not grooved 89. 



88. Incisor groove near outer edge of incisor; nose short, its length (to be- 



532 



