16. Tail very flat, broad and hairless; toes of hind feet with evident webs; 



claw of second toe double; adults somewhat over three feet in total 

 length (the largest native rodent) ; generally distributed over the 

 United States, but almost exterminated in many localities; Family 

 Castoridae 



Castor canadensis Kuhl Beaver 

 Not so 17. 



17. Body and short tail bristling with large, stiff quills; adults about two to 



three feet in total length; Family Erethn.onudae Porcupines 18. 



Not so 19. 



18. Hairs whitish at tips; New England to Penna. and westward through the 



Great Lakes Region 



Erethizon dorsatinn (Linn.) Canada Porcupine 

 Hairs with greenish-yellow tips; westward and south Vv'ard from N. D. 

 Ereth}zon epjxanthum Brandt Yellow-haired Porcupine 

 (Now considered to be a subspecies of the preceding) 



19. Tail so short that the mammal appears superficially to be without a tail; 



Pacific states; Family Aplodontndae 



Aplodontia rufa (Raf.) Mountain Beaver, Sewellel 

 With an evident tail 20. 



20. With four or more cheek teeth (premolars and molars) on each side of 



each jaw 21. 



With three grinding teeth on each side of each jaw 112. 



21. Tail thinly haired or almost bare 22. 

 Tail bushy or at least well covered with hair; Family Sciuridae 23. 



22. With fur-lined cheek pouches opening on each side of the mouth but not 



connecting with the mouth 63. 



No such pouches 112. 



23. Body very heavy set; tail short, less than one -fourth of total length, in 



most cases Marmots and Prairie Dogs 24. 



Body more slender; tail usually, but not in all cases, longer Chipmunks, 



Squirrels, Ground Squirrels, etc. 29. 



24. Thumb small, with flat nail; tail without a conspicuous black or white tip 



25. 

 Thumb with a claw, the same as the fingers; tail with a black or white tip 



27. 



25. Upper parts largely grizzled black and white; northwestern U. S. to 



Montana 



Marmota caligata (Eschscholtz) Hoary Marmot, Whistler 

 Upper parts reddish-brown 26. 



26. Under parts buff to brown; northern states 



Marmota monax (Linn.) Woodchuck, Groundhog, Marmot 

 Under parts yellowish to light brown; with a light band crosswise before 

 the eyes; in the Rockies and westward 



Marmota flaviventris (Audubon & Bachman) 

 Yellow-bellied Marmot 



506 



