130 Order RODENTIA 



14. Upper molars with crowns having rows of tubercules or 



cusps, fig. 74a, c 15 



Upper molars with crowns having enamel loops, fig. l\d, e, 

 rather than tubercules or cusps 20 



15. Upper incisors ^vith longitudinal groove on front surface or 



face of each harvest mice, Reithrodontomys spp. 



Upper incisors without grooves on faces 16 



16. Upper incisors, viewed from side, each with a pronounced 



terminal notch, fig. 74/ house mouse, Mus musculus 



Upper incisors, viewed from side, without terminal notches 17 



17. Infraorbital plate with anterior margin straight, fig. 74^ 



golden mouse, Peromyscus nuttalli 



Infraorbital plate with anterior margin bowed forward, as 

 in fig. 74/r 18 



18. Zygomatic width of skull anteriorly less than 12 mm. (i/> 



in.) deer mouse, Peromyscus manic id at us 



Zygomatic width of skull anteriorly more than 12 mm. 19 



19. Maxillary tooth rows each 4 mm. long or less 



white-footed mouse, Peromyscus leucopus 



Maxillary tooth rows each usually more than 4 mm. long . 

 cotton mouse, Peromyscus gossypinus 



20. Upper incisors each with longitudinal groove on face or - 



front surface, fig. 74/ 



southern bog lemming, Synaptomys coopen 



Upper incisors without grooves on faces 21 



21. Interorbital region more than 4 mm. w'dz 



pine vole, Pitymys pinetorum 



Interorbital region less than 4 mm. wide 22 



22. Last upper molars each with 5 or 6 enamel loops, fig. 7Ad. 



meadow vole, Microtus pennsyh'anicus 



Last upper molars each with 4 enamel loops, fig. 74r 



prairie vole, Microtus ochrogaster 



23. Upper molars with irregular triangles on crowns, fig. 74/» 



eastern wood rat, Neotoma floridana 



Upper molars with rows of cusps on crowns, fig. 74a, c 24 



24. Upper cheek teeth with cusps in 2 rows; skull less than 



35 mm. (1^ in.) long 25 



Upper cheek teeth with cusps in 3 rows; skull more than 

 35 mm. long 26 



25. Anterior half of rostrum wider than least interorbital space 



hispid cotton rat, Sigmodon hispid us* 



Anterior half of rostrum narrower than least interorbital 

 space rice rat, Oryzomys palustris 



26. Length of each parietal, measured along temporal ridge, 



less than greatest distance between temporal ridges 



roof rat, Rattus rattus 



Length of each parietal, measured along temporal ridge, 



about equal to distance between temporal ridges 



Norway rat, Rattus norvcgicus 



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



