Microtus chrotorrhmns (Miller) Rock Vole 

 Snout not distinctly yellowish; western states 176. 



176. Total length about five to si.x inches; tail short, less than one-third total 



length 177. 



Total length about seven to nine inches; tail about one-third total length 



178. 



177. Color above grayish; ears small, almost concealed by the fur; Rocky 



Mountain and Pacific states 



Microtus montanus (Peale) Mountain Vole 

 Color above brownish; ears plainly visible; Cal. and Oregon 

 Microtus calif ornicus (Peale) California Vole 



178. Tail blackish, scarcely lighter below; Pacific states 



Microtus townseyidii (Bachman) Townsend Meadow Mouse 

 Tail gray below 179. 



179. Total length of adult over eight inches; brown above; southward to Colo. 



Microtus richardsoni (De Kay) Water Rat 

 Total length of adult seven to eight inches; grayish-brown above, often 

 with a brighter mid-dorsal area; generally distributed over the western 

 states 



Microtus longicaudus (Merriam) Long-tailed Meadow Mouse 

 (Includes M. mordax (Merriam) ) 



KEY TO THE SKULLS OF THE PRINCIPAL GENERA OF MAMMALS 



1. With no incisor or canine teeth, and with cheek teeth peg-like 



Dasypus Armadillo 

 With incisors, or with cheek teeth showing definite grinding surfaces — 

 if skull is less than four inches long, take choice 15; if longer, take 2. 



2. With no upper incisors; often with knobs, horns or antlers on frontal 



bones (above the orbits) ; cheek teeth with crescents on grinding sur- 

 face 3 . 

 Upper incisors present; never with horns or antlers 12. 



3. Skull about two feet long, broad and flattened on top 4. 

 Smaller, rarely over eighteen inches long; cranium rounded and usually 



widest just behind the orbits 6. 



4. Parietal bone showing only on back of skull; space from tip of upper 



jaw to tip of nasals only about one-fifth of total length, tip of nasals 

 coming above a point halfway between tip of jaw and first cheek teeth 

 Bos Cow 

 Parietal bone showing on top of skull; space from tip of upper jaw to 

 tip of nasals about one-third of total length of skull, tip of nasals com- 

 ing above the first cheek teeth 5. 



5. Width of head between orbits more than half the length of the skull; 



horns present 



Bison Buffalo 

 Width of head not much more than one-third the length; males with 

 palmate antlers during most of year 



Alee Moose 



523 



