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_.] =Romerolagus diazi= (Diaz) Volcano Rabbit 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). Genus SYLVILAGUS Gray--Cottontails and Allies Revised by Nelson, N. Amer. Fauna, 29:58-158, August 31, 1909. 1867. _Sylvilagus_ Gray, Ann. and Mag. Nat. Hist., 20 (ser. 3):221. Type, _Lepus sylvaticus_ Bachman, _Lepus nuttalli mallurus_ Thomas. Total length, 291-538; tail, 18-73; hind foot, 71-110; ear from notch (dry) 41-74. Grayish to dark brownish above and lighter below; sutures of interparietal bone distinct throughout life; second to fourth cervical vertebrae broader than long with dorsal surface flattened and without carination.