SCIURID4—SCIURUS. 667 
and in pattern of coloration; from the Jarger, large-eared, long-, broad- 
tailed Spermophiles in the shorter, broader, more expanded form of the 
skull, its more convex dorsal outline, slenderer and less diverging zygomatic 
arches, the strict parallelism of the inner edges of the molar series, and the 
narrower and more anteriorly situated anteorbital foramina, etc. 
A division of the American Sciuri into other than arbitrary or artificial 
groups seems almost impossible. Although some of the species have two 
premolars and others one, some tufted and others tuftless ears; although 
some are of large size and others small; and although the skull varies in 
respect to convexity, interorbital breadth, and somewhat in other features, 
these differences are so variously combined that no sharply dividing lines can 
be drawn if more than a single character be taken. In respect to the pres- 
ence and size of the first upper premolar, the species fall into three sections, 
with which, to some extent, other characters correspond. In some of the 
species, so far as I can determine, there is but a single premolar at any period 
of life; in one (S. hudsonius and its varieties), a second is generally present, 
though often disappearing late in life; it is, however, so minute as to easily 
escape observation, never rising to the level of the other teeth, it being gen- 
erally merely a slender, minute point situated close to the inner edge of the 
anterior root of the second premolar, beneath the front edge of the base of its 
crown. Sometimes it is a mere point, and at other times is about one-half 
the height of the second premolar. In the greater number of the species, two 
premolars are permanently developed, but the first is often minute, scarcely 
reaching the height of the second, and is occasionally absent in those species 
in which, as a rule, it is present. 
The pelage varies greatly in character in the different species, being 
sometimes full, fine, and soft, even among the tropical forms; again, it is short 
and sparse, and at other times long, coarse, and stiff. 
Perhaps the most obviously variable feature is the character of the tail. 
Generally, its length, measured from the base to the end of the vertebra, is 
rather less than the length of the head and body; sometimes these two. 
measurements are equal, while occasionally the tail is a third less than the 
length of the head and body. The tail, to the end of the hairs, is very 
generally equal to or longer than the head and body; sometimes much longer, 
occasionally about equal, and more rarely somewhat less. The tail also varies 
greatly in fulness and breadth. In Sciurus fossor and S. colliei, the tail, 
when fully spread, has a breadth of five or six inches, the hairs of the sides 
