Order RODENTIA 



Tail not flattened dorsoventrally, and less than 25 mm. (1 

 in.) wide; head+body less than 400 mm. long 11 



11. Tail at least I1/3 times the length of head+body, fig. 102; 



upper jaw with 4 cheek teeth on each side 



meadow jumping mouse, Zapus hudsonius 



Tail less than I'/s times the length of head+body; upper 

 jaw with 3 cheek teeth on each side 12 



12. Tail flattened laterally; front feet each with 5 clawed toes 



muskrat. Ondatra zibethicus 



Tail not flattened; front feet each with 4 clawed toes 13 



13. Tail less than Vs length of head+body 14 



Tail more than '/s length of head+body 17 



14. Upper incisors each with shallow longitudinal groove on 



face, fig. 74/ southern bog lemming, Synaptomys cooperi 



Upper incisors not grooved 15 



15. Tail less than 25 mm. (1 in.) long; hind foot less than 18 



mm. (almost ^ in.) long; ears hidden in soft fur 



pine vole, Pitymys pinctorum 



Tail more than 25 mm. long; hind foot usually more than 

 18 mm. long; ears projecting above coarse fur 16 



16. Fur of under parts tipped with white; feet black; each last 



upper molar with 5 or 6 enamel loops, fig. I'^d 



meadow vole, Microtus pennsylvanicus 



Fur of under parts ochraceous; feet brown; each last up- 

 per molar with only 4 enamel loops, fig. 74^ 



prairie vole, Microtus ochrogaster 



Upper incisors each with a longitudinal groove 18 



Upper incisors not grooved 19 



Body having sides washed with fulvous; back brown 



western harvest mouse, Reithrodontomys megalotis 



Body having sides and back rich brown 



eastern harvest mouse, Reithrodontomys humilis* 



Over-all length of animal more than 200 mm. (8 in.) 20 



Over-all length of animal less than 200 mm. 24 



Fur on throat with each hair white its entire length; upper 

 cheek teeth with triangles but without distinct cusps, 



fig. 74^ eastern wood rat, Neotoma floridana 



Fur on throat with each hair white distally but darkened at 

 base; upper cheek teeth with distinct cusps rather than 

 triangles, fig. 74a, c 21 



1. Hind foot not more than 34 mm. (13/^ in.) long; under side 

 of tail light; cusps on upper cheek teeth in 2 rows 22 



Hind foot more than 34 mm. long; under side and upper 

 side of tail gray; cusps on upper cheek teeth in 3 rows . 23 



2. Fur long, grizzled butf and black; ears almost hidden in fur 

 hispid cotton rat, Sigmodon hispidus* 



Fur short, brown above and not grizzled ; ears projecting 

 above fur rice rat, Oryzomys palustris 



3. Tail shorter than length of head+body 



Norway rat, Rattus norvegicus 



*This species may occur in Illinois, but there are no official records of it here. 



J 



