Order RODENTIA 127 



Tail as long as or longer than head+body 



roof rat, Rattus rattus 



24. Belly and feet dirty gray or olive, house mouse, Mus musrulus 

 Belly and feet white 25 



25. Head, ears, back, and sides of a uniform reddish color . . 



golden mouse, Peromyscus nuttallt 



Head, ears, and back brown or black; sides variable and 

 lighter in color 26 



26. Hind foot 18 mm. {Y\ in.) or less in length; tail usually 



less than 60 mm. (2^ in.) in length; middorsal dark 



stripe prominent deer mouse, Peromyscus maniculatus 



Hind foot more than 18 mm. in length; tail usually more 

 than 60 mm. in length; middorsal dark stripe present or 

 absent 27 



27. Hind foot usually less than 23 mm. {% in.) in length; 



head+body usually less than 100 mm. (4 in.) in length 



white-footed mouse, Peromyscus leucopus 



Hind foot usually 23 mm. or more in length; head+body 



usually more than 100 mm. in length 



cotton mouse, Peromyscus gossypinus 



Skulls 



1. Each frontal bone with well-developed postorbital process, 



figs. 71a, b, e, 74/, k 2 



Each frontal bone without postorbital process 9 



2. Postorbital processes large, projecting at right angles to the 



longitudinal axis of the skull, fig. 73^; incisors whitish 



woodchuck, Marmota monax 



Postorbital processes small and angled backward ; incisors 

 yellowish 3 



3. Interorbital region deeply notched anterior to each post- 



orbital process, fig. 74; 



southern flying squirrel, Glaucomys volans 



Interorbital region not deeply notched anterior to each post- 

 orbital process, figs. 71a, 74-k 4 



4. Zygomatic arches tending to converge anteriorly, fig. 73a.. 5 

 Zygomatic arches nearly parallel, fig. 74k 7 



5. Nasals more than 17 mm. (?8 in.) long 



Franklin's ground squirrel, Citellus franklinii 



Nasals less than 17 mm. long 6 



6. Upper jaw with 5 cheek teeth on each side 



. . . thirteen-lined ground squirrel, Citellus IridecemlineatUi 



Upper jaw with 4 cheek teeth on each side 



eastern chipmunk, Tamias striatus 



7. Upper jaw with series of cheek teeth on each side less 



than 10 mm. (}£ in.) long 



red squirrel, Tamiasciurus hudsonu twf 



Upper jaw with series of cheek teeth on each side more than 

 10 mm. long 8 



iThis species probably is not now present in Illinois. 



