126 Order RODENTIA 



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

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



11. Tail at least l 1 /?, 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 H/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 1/3 length of head+body 14 



Tail more than 1/3 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 ^4 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. l\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 



17. Upper incisors each with a longitudinal groove 18 



Upper incisors not grooved 19 



18. Body having sides washed with fulvous; back brown 



western harvest mouse, Rcithrodontomys megalotis 



Body having sides and back rich brown 



eastern harvest mouse, Rcithrodontomys humilis* 



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



Over-all length of animal less than 200 mm 24 



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



cheek teeth with triangles but without distinct cusps, 



fig. 74£ eastern wood rat, Ncotoma ftoridana 



Fur on throat with each hair white distally but darkened at 

 base; upper cheek teeth with distinct cusps rather than 

 triangles, fig. lAa, c 21 



21. Hind foot not more than 34 mm. (\V% 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 



22. Fur long, grizzled buff 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 



23. 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. 



