'. Anterior extension of supraorbital process present, and posterior
extension free of braincase or leaving a slit between the process
and braincase.

6. Tympanic bullae large (see fig. 26).
_Sylvilagus audubonii_, p. 162

6'. Tympanic bullae small (see figs. 23, 25 and 27).

7. Restricted to Pacific coastal strip from Columbia River
south to tip of Baja California, west of Sierra
Nevada-Cascade Mountain Chain; hind foot less than 81.
_Sylvilagus bachmani_ and _S. mansuetus_, pp. 143, 147

7'. East of the Pacific coastal strip mentioned in 7; hind
foot usually more than 81.

8. If north of United States-Mexican boundary:

9. In Arizona, New Mexico and southern Colorado
posterior extension of supraorbital process free of
braincase, and supraoccipital shield posteriorly
pointed; from central Colorado north into Canada
diameter of external auditory meatus more than crown
length of last three cheek-teeth
_Sylvilagus nuttallii_, p. 161

9'. In Arizona, New Mexico and southeastern Colorado
posterior extension of supraorbital process of
frontal with its tip against, or fused to,
braincase, and supraoccipital shield posteriorly
truncate or notched; from central Colorado north
into Canada, diameter of external auditory meatus
less than crown length of last three cheek-teeth
_Sylvilagus floridanus_, p. 154

8'. If south of United States-Mexican boundary:

10. Geographic range restricted to Tres Marias Islands
_Sylvilagus graysoni_, p. 169

10'. Geographic range not including Tres Marias
Islands.

11. Underside of tail dingy gray or buffy (not
white).

12. Tail short (less than 30) and brown like rump;
ear from notch (dry) less