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 than 53; interorbital breadth less than 16. _Sylvilagus brasiliensis_, p. 141 12´. Tail of moderate length (more than 30) and dingy gray; ear from notch (dry) more than 53; interorbital breadth more than 16 _Sylvilagus insonus_, p. 168 11´. Underside of tail distinctly white. 13. Total length more than 476; ear from notch (dry) more than 64; interorbital breadth usually more than 19.3; geographic range, southwestern Mexico north of the Isthmus of Tehuantepec. _Sylvilagus cunicularius_, p. 169 13´. Total length less than 476; ear from notch (dry) less than 64; interorbital breadth usually less than 19.3; geographic range, Canada to Panamá _Sylvilagus floridanus_, p. 154 Genus ROMEROLAGUS Merriam--Volcano Rabbit