SCIURID2—SPERMOPHILUS. 823 
of Arctomys. As far as can be judged from the characters afforded by the few 
mandibular rami, from which as yet they are in the main only known, they 
more resembled, at least in dentition, the existing species of Arctomys and 
Sciurus than those of Cynomys, or the heavier-toothed forms of Spermophilus. 
As regards their present distribution, the Spermophili are confined mainly 
to the colder portions of the northern hemisphere, and are numerously and 
about equally represented in North America and the colder parts of the Old 
World. All the Old World forms, with one exception (S. eversmami of North- 
eastern Asia), are short-tailed forms, while the American species, with two 
or three exceptions, are long-tailed, and approach more nearly to Sciwrus and 
Tamias than do those of the Old World. Several of the species range as far 
westward as Central Europe, and are more or less numerous thence eastward 
to the eastern shores of Northern Asia. In North America, none occur east 
of the plains and prairies of the central portion of the continent, ranging 
thence westward to the Pacific coast. The genus has representatives from 
the shores of the Arctic coast southward to the plains of Northern Mexico. 
All of the known species, except one, have representatives within the United 
States, this being the S. annulatus, known as yet from a single specimen (of 
which the locality is authentic) from Colima, Western Mexico. Another, 
however (S. empetra), only just reaches our northwestern border, while no 
less than three (S. mezicanus, S. spilosoma, and S. grammurus) are known. to 
occur considerably beyond the northern boundary of Mexico. Only about 
one-fourth of the whole number of species are found at any one locality. 
It may be further noted that the pelage of all of the southern species 
consists of rather coarse, harsh, stiff hairs, generally flattened and grooved on 
the outer surface, with very little or no under fur, especially in summer ; 
while all the northern and alpine species, with one exception (S. franklini), 
have a soft, thick, furry pelage, with abundant under fur. In respect to 
geographical variation, there is, as a rule, a decided decline in size southward 
in individuals of the same species, with quite often an appreciable increase 
in the size of the ears. In respect to seasonal variations, the pelage is every- 
where softer and heavier toward winter, with the soles quite fully clothed, 
while in summer they are generally more or less naked. All the northern 
species are known to pass the colder portion of the year in a state of hiberna- 
tion. Southward, the period of hibernation is of course much shorter, while 
the most southern forms continue mure or less active throughout the winter. 
