668 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
being two and a half to three inches in length. On the other hand, in S, 
hypopyrrhus, in which the length of the tail is fully as great as in S. fossor 
and S. colligi, its breadth is about one-third less. The tail is narrowest (in 
the long-tailed species) in S. @stuans, but even in this species it is still dis- 
tinctly flattened or distichous, except possibly in badly prepared specimens, 
though not unfrequently cylindrical at the end. 
M. F. Cuvier, in 1818, separated the Guerlinguets from the true Squirrels 
under the name Macroxus, referring to this group the Sciurus astuans of 
Brazil and the Sciwrus vittatus of India. The characters given for this group, 
as distinguishing it from Sciwrus, are the greater cerebral capacity of the skull, 
the tail not distichous at the point, the nasal portion of the skull separated 
from the cranial by a strong depression, and by the large size of the testes.* 
Although Macroxus was adopted by Lesson in 1827, it has been by most 
writers properly ignored, the characters given having little value. The 
Macrorus of Gray (1867), although including the species referred to Macrozus 
by Cuvier, is based on a wholly different feature, namely, on the absence 
of ear-tufts, and hence embraces all the Sciwri with untufted ears. Scirus, 
as left by Gray, thus includes only a few species with conspicuously tufted 
ears, like S. vulgaris and S. aberti. This distinction, it is almost needless 
to say, is not of the slightest importance as a generic character, the long ear- 
tufts being merely a seasonal feature, if not in part, in some species at 
least, simply individual. In the North American S. aberti, individuals are 
found both with and without them at the same localities and seasons, while 
in other individuals they may be found of different degrees of development. 
Many other species have the ears slightly tufted in winter, which are wholly 
without ear-tufts in summer. ; 
The American Sciwri reach their greatest numerical development in 
Middle America; Southern Mexico and Central America being far richer in 
species than any other region in either North or South America of similar 
area. ‘The species are here remarkable also for their extreme variability in 
color. Melanistic phases of coloration are frequent, while all the species are 
normally fulvous, orange, or red below, white-bellied specimens from this 
region being exceptional; with perhaps a single exception, none of the specific 
*The diagnosis given by Cuvier of “les Guerlinguets” (Macrozus) is as follows :—“ Ce sont des 
écureuils dont la queue n’est point distingué et dont la capacité cérébrale surpasse de beaucoup celle 
des écureuils proprement dits et celle des tamias. Une dépression trés-marquée sépare le crane d’un 
museau peu alongé. Entin, ils sont xemarquables par leurs testicules volumineux.”—( Dict. Sci. Nat., lix, 
1829, 474.) 
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