1´. Antorbital extension of supraorbital process less than ½ of posterior extension or entirely absent; first upper cheek-tooth with more than one (usually 3) re-entrant angles on anterior face; re-entrant angle of second upper cheek-tooth crenate. 2. Anterior extension of supraorbital process absent (or if a point is barely indicated, then 5/6 or all of posterior process fused to braincase). 3. Tympanic bulla smaller than foramen magnum; hind foot more than 74; geographic range wholly in United States. 4. Ear more than 58 from notch in dried skin; basilar length of skull more than 63 _Sylvilagus aquaticus_, p. 166 4´. Ear less than 58 from notch in dried skin; basilar length of skull less than 63. 5. Underside of tail white; posterior extension of supraorbital process tapering to a slender point, this point free of braincase or barely touching it and leaving a slit or long foramen _Sylvilagus transitionalis_, p. 160 5´. Underside of tail brown or gray; posterior extension of supraorbital process always fused to skull, usually for entire length but in occasional specimens there is small foramen at middle of posterior extension of supraorbital process _Sylvilagus palustris_, p. 147 3´. Tympanic bulla as large as foramen magnum; hind foot less than 74; geographic range limited to southern edge of Mexican tableland at high elevations _Romerolagus diazi_, p. 138 2´. 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: