View source for Key to the Reithrodontomys of the USA

This key to the harvest mice (''Reithrodontomys'') of the United States is borrowed from Spencer & Cameron (1982).

Spencer SR, Cameron GN. 1982. ''Reithrodontomys fulvescens''. Mammalian species (174): 1-7.

==1==
* First primary fold of M3 at least as long as second primary fold, each usually extending more than halfway across crown; worn occlusal surface of left m3 S-shaped.... ''[[Reithrodontomys fulvescens]]''
* First primary fold of M3 distinctly shorter than second primary fold, extending less than halfway across crown; worn occlusal surface of left m3 C-shaped {{cl|2|2}}

==2==
{{cl|1}}
* Distinct labial ridge, often with cusplets, on m1 and m2; major fold and 2nd primary fold in M1 and M2 tending to coalesce, isolating anterior cusps from posterior cusps; occurring in southeastern USA... ''[[Reithrodontomys humulis]]''
* No distinct labial ridge on m1 and m2; major fold and second primary fold in M1 and M2 meeting but not coalescing, and thus isolating anterior cusps from posterior cusps; not occurring in southeastern USA {{cl|3|3}}

==3==
{{cl|2}}
* Breadth of braincase not exceeding 9.6; rostrum short and broad; tail shorter than head and body; occurring in Great Plains and southwestern USA.... ''[[Reithrodontomys montanus]]''
* Breadth of braincase more than 9.5; rostrum longer and narrower; tail approximately equal to, or longer than head and body {{cl|4|4}}

==4==
{{cl|3}}
* Fur of upperparts long, dense, heavily pigmented; ears blackish; occurring in salt marshes in vicinity of San Francisco Bay, California... ''[[Reithrodontomys raviventris]]''
* Fur of upperparts shorter and less dense, varying from pale buff to reddish brown; ears buffy or fuscous; not occurring in salt marshes in vicinity of San Francisco Bay, California... ''[[Reithrodontomys megalotis]]''

[[Category:Key]]
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