1'. Antorbital extension of supraorbital process less than 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 pal.u.s.tris_, 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:

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 florida.n.u.s_, 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 Panama _Sylvilagus florida.n.u.s_, 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 gra.s.s, these small rabbits are mainly nocturnal and crepuscular, but sometimes are active by day, especially in cloudy weather in the period of mating.

[Ill.u.s.tration: FIG. 6. Distribution of _Romerolagus diazi_.]

=Romerolagus diazi= (Diaz)

Volcano Rabbit

1893. _Lepus diazi_ Diaz, Catal. Com. Geograf.-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, Mexico.

_Range._--Canadian Life-zone of the mountains bounding the eastern, southern and western sides of the Valley of Mexico. _Marginal records._--Mexico: Monte Rio Frio, 45 km. ESE Mexico City (Davis, 1944:401). Puebla: type locality. Mexico: Mt. Popocatepetl (Nelson, 1909:280). Distrito Federal: 31 km. S Mexico City (30815 KU). Mexico: Llano Grande, 3 km. W Tlalma.n.a.lco (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.

The delectable flesh of members of this genus, the large numbers that occur on a small area, even in thickly settled rural areas, and the wariness that rabbits soon develop when much hunted, give them top ranking among small game mammals. Tens of thousands of cottontails in Kansas and Missouri (_Sylvilagus florida.n.u.s_ and some _S. audubonii_) are captured alive, transported to the eastern United States and released there to bolster the local supply of game. Considering that certain ectoparasites are limited to certain hosts and that some ectoparasites transmit such diseases as Rocky Mountain Spotted Fever whereas other ectoparasites do not, this transplantation of rabbits is dangerous. Also, expenditure of $100.00 on improving the habitat for _Sylvilagus_ in a given area in the eastern United States would produce more cottontails than the expenditure of the same sum for live animals, from the Middlewest, that are to be released (see Langenbach and Beule, 1942:14, 15 and 30).

Different species venture different distances from cover to feed. The Audubon cottontail of west-central California ventures a hundred feet and more from cover but the brush rabbit was never seen (Orr, 1940:182) farther than 42 feet from cover. In the thirties, when a gladiolus farmer from the chaparral belt of Santa Clara County, California, visited the University of California seeking advice on how to prevent damage by "cottontails" to his gladioli plantings, we asked the farmer if brush rabbits or cottontails were responsible and suggested to the farmer, who was unable to distinguish between the two, that an animal be killed and submitted for identification. When this was done, the brush rabbit (_Sylvilagus bachmani_) was found to be responsible for the damage. Robert T. Orr's recommendation that the chaparral (brush) be cut back 45 feet from the gladioli plantings was reluctantly followed and proved to be effective. A letter from a Santa Clara County agricultural official a couple of years later expressed thanks for the recommendation made by Orr, and estimated that adoption of his recommendations saved farmers of that one county $40,000 annually. This incident ill.u.s.trates how detailed knowledge of the life history of a given kind of animal and control of its environment, rather than direct "control" of the animal, is sometimes of value to man.

The genus _Sylvilagus_ is restricted to the New World; the two species _Sylvilagus brasiliensis_ and _S. florida.n.u.s_ are the only two which occur in South America and they occur also in North America.

Subgenus BRACHYLAGUS Miller--Pigmy Rabbit

1900. _Brachylagus_ Miller, Proc. Biol. Soc. Washington, 13:157, June 13. Type, _Lepus idahoensis_ Merriam. For characters see subgenus _Sylvilagus_.