E. Raymond Hall

“ 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. ”

“ The cottontails with equally small tympanic bullae from more western and more southern localities are referable to Sylvilagus floridanus holzneri on the basis of a posteriorly truncate or emarginate supraoccipital shield and a narrower and shorter space (usually a "foramen") between the braincase and the posterior extension of the supraorbital process. In S. f. holzneri the posterior end of the posterior process fuses with the braincase whereas the posterior end of this process in Arizonan specimens of S. n. pinetis merely lies against the braincase or projects free of it. ”

“ Because some mammalogists have suspected that intergradation between Sylvilagus floridanus similis and Sylvilagus nuttallii grangeri occurs along the eastern base of the Rocky Mountains, we have examined specimens which may throw light on this matter.
From S. f. similis (holotype and three topotypes) , S. n. grangeri (eight practical topotypes from Redfern, South Dakota) differ as follows: Throat patch darker; hind foot shorter; ear (dry) from notch longer; rostrum narrower; posterior extension of supraorbital process enclosing a longer and wider space between it and the braincase ”