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
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
1896. _Romerolagus_ Merriam, Proc. Biol. Soc. Washington, 10:173, December 29. Type, _Romerolagus nelsoni_ Merriam = _Lepus diazi_ Diaz.
Total length 300 to 311; tail rudimentary; hind foot, 52; ear from notch (dry), 36; upper parts grizzled buffy brown or dull cinnamon brown; underparts dingy gray; anterior projection of supraorbital process absent; jugal projecting posteriorly past squamosal root of zygomatic arch more than half way to external auditory meatus. The two cranial characters mentioned are resemblances to pikas although the skull otherwise resembles that of the true rabbits. The genus contains only the one living species.
Living in well defined runways in the dense sacoton grass, these small rabbits are mainly nocturnal and crepuscular, but sometimes are active by day, especially in cloudy weather in the period of mating.
[Illustration: FIG. 6. Distribution of _Romerolagus diazi_.]
1893. _Lepus diazi_ Diaz, Catal. Com. Geográf.-Expl. Repub. Mex. Expos. Internac. Columb. Chicago, pl. 42, March, 1893, type from eastern slope of Mount Ixtaccihuatl, Puebla.
1911. _Romerolagus diazi_ Miller, Proc. Biol. Soc. Washington, 24:228, October 31, 1911.
1896. _Romerolagus nelsoni_ Merriam, Proc. Biol. Soc. Washington, 10:173, December 29, 1896, type from west slope Mount Popocatepetl, 11,000 feet, México.
_Range._--Canadian Life-zone of the mountains bounding the eastern, southern and western sides of the Valley of Mexico. _Marginal records._--México: Monte Río Frío, 45 km. ESE Mexico City (Davis, 1944:401). Puebla: type locality. México: Mt. Popocatepetl (Nelson, 1909:280). Distrito Federal: 31 km. S Mexico City (30815 KU). México: Llano Grande, 3 km. W Tlalmanalco (28278 KU).