A Key to the Genera of North American Mammals 



FFFF. Form goat-like, hair long, pure white . . OREAMNOS, 57 

 EE. Horns hollow, forked, and shed periodically; teeth, i , c $, 



p f, m f ANTILOCAPRA, 54 



EEE. Horns solid branching antlers, which are shed periodi- 

 cally ; teeth, i , c {j $, p f , m f. 



F. Nose completely covered with hair .... RANGIFER, 47-54 

 FF. Nose not completely haired. 



G. Antlers broadly palmate or flattened. .. .ALCES, 43 

 GG. Antlers not flattened. 



H. Animals medium or small; antlers not more than 



twenty-five inches long ODOCOILEUS, 34-43 



HH. Animals large; antlers four to five feet. 



CERVUS, 31-34 

 CC. Toes terminating in claws. 



D. A gap on each side of the jaws caused by the absence of canine 

 teeth. Incisors two in each jaw, large and protruding, 

 working against each other like a pair of chisels. 

 E. A pair of small rudimentary incisors behind the upper pair 



of large ones. 

 F. Tail short; hind legs much longer than the front ones; 



teeth, if, c $, p -f, m f LEPUS, 75-92 



FF. No external tail; legs about equal; p f . . . . OCHOTOXA, 93 

 EE. No extra incisors. 



F. Hair interspersed with sharp spine-like quills; teeth, i j, c g, 



p $, m ERETHIZON, 94 



FF. Without spines. 



G. Form mole-like; fore feet modified for digging; no 

 external ear; teeth, i |, c , p $, m |. 



H. Incisors grooved GEOMYS, 97-99 



HH. Incisors not grooved THOMOMYS, 96 



GG. Form not mole-like; fore feet normal. 



H. Body heavy and thick-set, 12-20 inches long exclu- 

 sive of tail; legs short. 



I. Tail very broad, flat, covered with scales; teeth, 



i T. c $> P 1> m I CASTOR, 145-150 



II. Tail long and narrow, flattened vertically, and 



nearly naked teeth, i |, c $, p $, m f . 



FIBER, 121-127 



III. Tail hairy ; teeth, i $, c fl, p f { m |. 



J. Tail very short, less than the head. 



APLODONTIA, 150 

 JJ. Tail moderate, longer than the head, 



ARCTOMYS, 151-159 



HH. Form mouse-like or rat-like; size not larger than a 

 common rat (body less than 10 ins. without 

 tail) ; teeth usually, i j, c $, p $, m f. 



I. Pouches on the sides of the face opening near the 



mouth. 



J. Tail very long; teeth as above. PERODIPUS, 100 

 JJ. Tail moderate; teeth, i j, c , p }, m |. 



PEROGNATHUS, 100 



II. No external cheek pouches. 



J. Hind legs much longer than the front ones; 



tail exceeding head and body; teeth in one 



species, i -J-, c $, p {, m |. . . .ZAPUS, 102-105 



JJ. Hind legs not markedly longer than front ones. 



K. Thick-set, short-legged, short -eared mice of 



the meadow mouse type. 



L. Tail less than one-third the length of 

 head and body, generally much less. 



298 



