1914.] 



KEY TO SPECIES AND SUBSPECIES. 



ff. Tail distinctly bicolor. 



g. Pelage long; upper molars always with subsidiary enamel loops 

 (except in colimse); habitat above 9,000 feet altitude. 



h. Braincase flattened perotensis (p. 69). 



hh. Braincase globular. 



i. Larger; tail length more than 95 mm. 



j. Breadth of braincase more than 11 mm chrysopsis (p. 66). 



jj. Breadth of braincase less than 11 mm orizabx (p. 69). 



ii. Smaller; tail length less than 95 mm colimse (p. 59) 



gg. Pelage short; upper molars usually without subsidiary enamel loops; 



habitat below 9,000 feet altitude. 



Ji. Skull length less than 22 mm zacatecx (p. 39). 



hh. Skull length more than 22 mm. 



i. Larger; hind foot 20-21 mm. 



j. Braincase flattened luteolus (p. 57). 



jj. Braincase rounded nerterus (p. 60). 



ii. Smaller; hind foot 18-20 mm. 



j. Nasals longer (8-9.7 mm.); black dorsal area well defined. 



Ic. Skull larger, with wide? zygomata dorsalis (p. 61). 



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

 jj. Nasals shorter (7.3-8.8 mm.); black dorsal area not well defined. 



I. Tail longer (88-101 mm.); (Mexico) difficilis (p. 50). 



JcJc. Tail shorter (82-92 mm.) ; (Costa Rica) australis (p. 62). 



dd. Ears light brown or drab, with ochraceous hairs. 

 e. Underparts buffy or ochraceous. 

 /. Skull length more than 22.5 mm. 



g. Length of nasals more than 8.5 mm toltecus (p. 51). 



gg. Length of nasals less than 8.5 mm levipes (p. 64). 



ff. Skull length less than 22.5 mm. 



g. Tail length 105 mm. or more .mustelinus (p. 54). 



gg. Tail length less than 105 mm. 

 h. Upperparts strongly mixed with blackish. 



i. Underparts strongly washed with buff difficilis (p. 50). 



ii. Underparts faintly washed with buff aurantius (p. 48). 



hh. Upperparts mainly ochraceous-buff. 



i. Larger and paler tenuis (p. 45). 



u. Smaller and brighter nelsoni (p. 53). 



ee. Underparts white. 

 /. Tail length more than 104 mm. 



g. Larger; skull length 23 mm. or more toltecus (p. 51). 



gg. Smaller; skull length less than 23 mm helvolus (p. 52). 



ff. Tail length less than 104 mm. 



g. Skull length less than 20 mm .amcenus (p. 55), 



gg. Skull length more than 20 mm. 

 h. Tail length more than 80 mm. 



i. Upperparts tawny-ochraceous, or cinnamon mixed with black. 



j. Braincase flatter; nasals shorter chiapensis (p. 53). 



jj. Braincase higher; nasals longer aurantius (p. 48). 



ii. Upperparts ochraceous-buff or mixed with blackish. 

 j. Upperparts mainly ochraceous-buff. 



Jc. Sides deep ochraceous-buff tenuis (p. 45). 



kk. Sides pale ochraceous-buff, 

 28657°— 14 2 



