The specimen (W.D. Hollister, original No. 208) from the Datil Mountains, lent to us by the Colorado Museum of Natural History, does have, as Nelson (op. cit.) pointed out, larger tympanic bullae and a slenderer rostrum than do other specimens of S. f. cognatus. Nevertheless, No. 208, agrees with cognatus and differs from Sylvilagus nuttallii pinetis in the greater vertical depth of the zygoma, the greater transverse width of the first pair of upper incisors, the broader posterior extensions of the supraorbital processes, the fusion (instead of freedom from, or mere touching to, the braincase) of the tips of these extensions, the less upturned supraorbital processes, and the more nearly truncate posterior margin of the supraorbital shield. Therefore, the specimen is referable to Sylvilagus floridanus cognatus. The slender rostrum and large tympanic bullae of No. 208 are either individual variations or features peculiar to the population of Sylvilagus floridanus in the Datil Mountains.
Sylvilagus floridanus robustus Bailey
1905. Lepus pinetis robustus V. Bailey, N. Amer. Fauna, 25:159, October 24.
Nelson (N. Amer. Fauna, 29:194–195, August 31, 1909) described specimens from the Big Bend area of Texas. This was the only area from which Nelson had specimens. Our examination of these same specimens indicates that his description of them was accurate. Davis and Robertson (Jour. Mamm., 25:271, September 8, 1944) recorded a specimen, under the name Sylvilagus robustus, from "The Bowl, Guadalupe Mountains, Culberson County, Texas." Our examination of the skull of this specimen (♀ adult, No. 658, Mus. Zool., Louisiana State University) indicates that it is, among named kinds of rabbits, best referred to robustus. The specimen is morphologically as well as geographically intermediate between S. f. cognatus and S. robustus. This morphological intermediacy is illustrated by certain of the following cranial measurements of three adult females: No. 108695 (robustus), Chisos Mts.; No. 658 from the Guadalupe Mts.; and No. 128651, NE slope Capitan Mts. Basilar length, 59.2, 54.2, 54.4; length of nasals, 33.9, 31.1, 32.2; breadth of rostrum above premolars, 19.3, 17.5, 17.0; depth of rostrum in front of premolars, 15.8, 14.8, 14.0; interorbital breadth, 20.4, 19.1, 19.7; parietal breadth, 27.2, 27.1, 26.5; diameter of bulla, 13.3, 12.2, 10.7. Considering the intermediate nature of specimen No. 648, and the kind and amount of difference between Sylvilagus floridanus cognatus and S. robustus, it seems appropriate to us to use the name-combination Sylvilagus floridanus robustus.
Actual intergradation, in the sense of interbreeding between individuals of a continuously distributed population of animals, probably does not occur regularly between S. f. cognatus and S. f. robustus nor between several populations within either one of these subspecies; in south-central Arizona and western Texas the animals are said to occur only in the higher parts of the mountains. Consequently a given population is separated from another by low-lying territory inhospitable to the species Sylvilagus floridanus. This low-lying territory is inhabited by another species, Sylvilagus audubonii. More intensive collecting in the region concerned may, however, show a continuous distribution of the species Sylvilagus floridanus in several areas where it seems now to have an interrupted distribution.
Sylvilagus audubonii neomexicanus Nelson
1907. Sylvilagus audubonii neomexicanus Nelson, Proc. Biol. Soc. Washington, 20:83, July 22.
Nelson (N. Amer. Fauna, 29:230, August 31, 1909) listed under Sylvilagus audubonii cedrophilus Nelson an adult female, skin with skull (U.S. Nat. Mus., Biol. Surv. Coll., No. 108698) from fifteen miles south of Alpine, Texas. Nelson (loc. cit.) remarked that the "bleached" color of the back and the great lateral breadth of the tympanic bullae of No. 108698 were peculiarities not possessed by any other specimen examined. Geographically, the locality of capture is far south of other known occurrences of S. a. cedrophilus and approximately on the boundary separating the range of S. a. minor from that of S. a. neomexicanus. The large size, which may have induced Nelson to refer the specimen to S. a. cedrophilus, is not surprising considering that the individual is a female and fully adult. A combination of new and old fur on the upper parts presents a pattern that might be duplicated in other specimens of S. a. neomexicanus. The lateral inflation of the tympanic bullae can be interpreted as intergradation with the geographically adjacent S. a. minor to the south; S. a. minor has large bullae. There are no features otherwise which suggest that the specimen is anything other than Sylvilagus audubonii neomexicanus and we refer it to that subspecies.