16 



NORTH AMEEICAN FAUNA. 



[No. 36. 



R. tenuirostris group: 



Reithrodontomys tenuirostris tenuirostris Mer- 



riam Todos Santos, Guatemala. 



tenuirostris aureus Merriam Calel, Guatemala. 



creper Bangs Volcan de Chiriqui, Panama. 



microdon microdon Merriam Todos Santos, Guatemala. 



microdon albilabris Merriam Cerro San Felipe, Oaxaca. 



Key to Species and Subspecies.^ 



[Based on adults.] 



a. Outer wall of anteorbital foramen decidedly broader than width of interpterygoid 

 fossa; upper molars usually without subsidiary enamel loops. ^ 

 &. Tail length, 120 mm. or more. 



c. Greatest length of skull more than 24 mm hirsutus (p. 66). 



cc. Greatest length of skull less than 24 mm. 



d. Dorsal area blackish toltecus (p. 51). 



dd. Dorsal area not blackish otus (p. 55). 



66. Tail length less than 120 mm. 

 c. Tail length more than 65 mm. 

 d. Ears blackish brown, without ochraceoua hairs. 

 e. Underparts whitish. 

 /. Tail length 100 mm, or more. 



g. Hind foot more than 20 mm tolucae (p. 68). 



gg. Hind foot less than 20 mm alleni (p. 59). 



/. Tail length less than 100 mm. 



g. Upperparts mainly ochraceous chiapensis (p. 53). 



gg. Upperparts mainly blackish or brownish. 

 Ti. Paler; sides light ochraceous-buff. 



i. Larger; hind foot more than 18 mm cinereus (p. 35). 



n. Smaller; hind foot less than 18 mm. 



j. Sides of face pure buff (Arizona) arizonensis (p. 38). 



jj. Sides of face ochraceous-buff or grayish. 



h. Tail length more than 80 mm. (Lower California) 



peninsulx (p. 35). 



hk. Tail length less than 80 mm. 



I. Feet gray (California) longicaudus (p. 33)- 



II. Feet white nigrescens (p. 32). 



hh. Darker; sides dark ochraceous-buff or pinkish cinnamon. 



i. Skull length less than 21.5 mm. 



j. Smaller; hind foot less than 17 mm amoles (p. 40). 



jj. Larger; hind foot more than 17 mm haliccetes (p. 42). 



a. Skull length more than 21.5 mm. 

 j. Tail length more than 80 mm. 



k. Breadth of braincase more than 11 mm tolucx (p. 68). 



kk. Breadth of braincase less than 11 mm. 



I. Skull larger, with wider zygomata dorsalis (p. 61). 



U. Skull smaller, with narrower zygomata, .saturatus (p. 36). 

 jj. Tail length less than 80 mm. 



k. Ear from notch more than 13 mm alticolus (p. 37). 



kk. Ear from notch less than 13 mm modestus (p. 63). 



ee. Underparts ochraceous-buff or pinkish cinnamon. 

 /. Tail unicolor or nearly so. 



g. Tail length more than 80 mm rufescens (p. 56). 



gg. Tail length less than 80 mm raviventris (p. 41). 



1 Color comparisons made with Ridgway's "Color standards and color nomenclature" (1912). 



2 Present in levipes, hirsutus, chrysopsis, tolucx, orUa'bx, and perotensis. 



