21. Back and sides the same color, under parts slightly lighter; Maine to Wis., 



southward to Georgia 



Sorex fumeus Miller Smoky Shrew 

 Tricolor, hack, sides and under parts of distinctly different shades; Maine 

 to S. D. 



Sorex arcticus Kerr Saddle-backed Shrew 



22. Third unicuspid tooth in upper jaw not smaller than the fourth 23. 

 Third unicuspid smaller than the fourth 25. 



23. Tail definitely hicolor, brown above, yellowish below; with wide distribu' 



tion, including the area from Wash, to N. D. and southward to 

 northern N. M. 



Sorex cmereus Kerr Masked Shrew 

 Tail not markedly bicolor; found in Oregon and Cal. 24. 



24. Total length about three and one-half inches; eastern Oregon 



Sorex preblei Jackson Preble's Shrew 

 Total length about four inches; Sierra Nevadas in Cal. 

 Sorex lyelli Merriam Mount Lyell Shrew 



25. Lower parts practically as dark as upper; tail sharply bicolor, whitish be 



low; Pacific states northward 



Sorex trowhridgii Baird Trowbridge Shrew 

 Lower parts lighter than upper; tail not sharply bicolor 26. 



26. Grayish; body over one and one-half times the tail length; southern Cal. 



Sorex ornatus Merriam California Shrew 

 Brownish; body less than one and one-half times the tail length; Rocky 

 Mountain states and Pacific coast into Cal. 27. 



27. About six inches in total length; coastal area of Oregon and northern Cal. 



Sorex pacificus Coues Pacific Shrew 

 About four and one-half inches in total length; Rocky Mountain states 28. 



28. Exposed part of upper front, and largest side, teeth about as deep as wide 



Sorex ohscurus Merriam Dusky Shrew 

 Exposed part of these teeth considerably wider than deep 

 Sorex vagrans Baird Wandering Shrew 



29. Tail long and almost hairless; with ten upper and eight lower incisor teeth 



(Marsupials) 30. 



Tail hairy; less than ten upper and eight lower incisors (Carnivores) 33. 



30. Mouse or rat size, total length seldom as much as fifteen inches (Not 



native to North America, but frequently accidently brought in with 

 fruit. Not keyed to species.) 



Marmosa Gray Mouse Opossums 

 Larger, total length about thirty inches in adult 31. 



3L Tail about one-third of total length; N. Y. to Wis. and Texas; also intro- 

 duced into Cal. 



Didelphis virgmiana virginiana Kerr Virginia Opossum 

 Tail about one-half of total length; Georgia to Texas 32. 



32. Color grizzled-gray; tail dark only at base; Ga. to La. 



Didelphis virginiana pigra Bangs Florida Opossum 



494 



