ginning of cheek'bone arch) one-fourth the total skull length 



Synaptomys Lemming Mouse 

 Incisor groove in center of face of incisor; nose about one-third of length 

 of skull 



Jsiapaeozapus Woodland Jumping Mouse 



89. Enamel folded on first cheek teeth in definite pattern of letter S; southern 



states 



Sigmodon Cotton Rat 

 Enamel folds not particularly S-shaped 90. 



90. Palate extending to or behind level of last cheek teeth; cheek teeth with 



two irregular rows of cusps; southern states 

 Oryzomys Rice Rat 

 Palate not extending as far back as the level of back of last cheek teeth; 

 cheek teeth with open or closed enamel triangles 91. 



91. Cheek teeth each with two small roots (in adult); embedded end of 



lower incisor begins just anterior to a small opening near back of jaw 

 and just posterior to root of last cheek tooth; northern states 92. 



Cheek teeth of same diameter throughout length; embedded end of lower 

 incisor longer, running almost to back of jawbone; generally dis' 

 tributed 93. 



92. Distinct raised line or ridge over top of eye-socket and extending back 



on braincase; cheek teeth large, the upper about one-half as wide as the 

 space between tooth rows; Canada, Rockies and West Coast in moun* 

 tains 



Phenacomys Tree Mouse, Lemming Mouse 

 Skull delicate, no ridge over eyc'socket; cheek teeth small, the upper 

 about one-fourth as wide as the space between tooth rows; northern 

 states 



Clethrionomys (Evotomys) Red-backed Mouse 



93. Zygomatic width about one-half the length of the skull; skull about two 



inches long 



Kleotoma Wood Rat 

 Zygomatic width definitely more than one-half the length of skull; skull 

 about one inch long (except in one large Florida species) 94. 



94. Skull about one inch long; length about one and two-thirds the zygO' 



matic width 



Microtus Meadow Mouse 

 Skull smaller, barely over % inch; short and wide, length one and one 

 half times the width 



Pitymys Pine Mouse 



95. Bullae so large as to be visible from above 96. 

 Bullae not extreme, not visible from above 98. 



96. Each cheek tooth with a U-shaped enamel fold 



Perognathus Pocket Mouse 

 Cheek teeth each with a simple outer margin of enamel 97. 



97. Maxilla with a large triangular projection before each orbit; each cheek 



tooth with a complete outer ring of enamel 



533 



