68. About two and one-half inches long; upper cheek teeth converging pos- 



teriorly; narrowest part of palate between last teeth; incisors whitish 

 Cynomys Prairicdog 

 Seldom over one and three-fourths inches; upper cheek teeth almost par- 

 allel; incisors yellow or orange 69. 



69. Width of nasals only about one-third of the distance between orbits 70. 

 Width of nasals about one-half or more of the distance between orbits 71. 



70. Total length of skull about one and one-half inches 



Citellns {Ammospermophilus) Antelope Ground Squirrel 

 Total length of skull about two inches 



Citellns (Otospermophilus) Rock Squirrel 



71. Nasals tapering posteriorly 



Citellus (Callospermophilus) Say's and Mantled Ground 

 Squirrel 

 Nasals scarcely tapering posteriorly 

 Citellus Ground Squirrel 



72. Cheek teeth on opposite sides of jaw converging and almost meeting an' 



teriorly; palate between molars abruptly lower than the anterior pof 



tion (step-like) 73. 



Cheek teeth in almost parallel rows; palate usually on one level or 



smoothly sloping 75. 



73. Skull two and one-half inches long; incisors white; molars sharply angled 



on sides 



Cavia Guinea-pig 

 Skull four to five inches long; incisors orange; cheek teeth not sharply 

 angled on sides 74. 



74. Space between orbits considerably wider than nasals; ante-orbital open- 



ing so large as to appear like an orbit 

 Erethizon Porcupine 

 Space between orbits about equal to width of nasals; ante-orbital open- 

 ing small 



Castor Beaver 



75. Skull about two and one-half inches long and compressed to a ridge be- 



tween orbits; ante-orbital opening large; incisors orange 

 Ondatra Muskrat 

 Not with the preceding combination of characters 76. 



76. Cheek teeth with rounded tubercles, in two or three rows 77. 

 Cheek teeth with plane surfaces or with enamel folds forming open or 



closed triangles (young animals may appear to have rounded tubercles 

 but rarely in rows) 85. 



77. Upper incisors grooved; skull seldom over one inch long 78. 

 Upper incisors not grooved; skull from % to 1^/2 inches long 80. 



78. Bullae much enlarged, forming bulges on the back corners of the skull; 



Dakotas and Neb. westward 



Perognathus Pocket Mouse 

 Bullae normal, not visible from above; generally distributed 79. 



531 



