SCIURIDZ—GENERAL CONSIDERATIONS. 639 
mophilus to Cynomys is almost again without hiatus. Arctomys is again not 
greatly different from Cynomys and some of the larger species of Spermophi- 
lus. Whatever character is taken, whether the general form, the dentition, 
the size of the ear, the character of the tail, the presence or absence of cheek- 
pouches, or even habits, and especially if all are taken collectively, no strong 
lines of demarcation can anywhere be drawn, and even the question of the 
number of properly recognizable genera is one of not easy solution. The 
division of the group into subfamilies, or “tribes”, is generally made between 
Tamias and Spermophilus, the former being associated with Xerus, Sciurus, 
Pteromys, and Sciuropterus to form a group Sciurine, or “True Squirrels”, 
and the latter (with its subdivisions) with Cynomys and Arctomys to form 
the subfamily, or “tribe”, Avctomyine, or “ Burrowing Squirrels”. I find, 
however, that here no line of separation can be drawn, Tamias and Spermo- 
vhilus so thoroughly intergrading that no feature serves to trenchantly separate 
them. <A much better hiatus occurs between Cynomys and Spermophilus, or 
even between Cynomys and Arctomys, or again between Sciuropterus and 
Sciurus. 
Between Pteromys, Sciuropterus, and Sciurus, the chief difference con- 
sists in the presence, in the two former, of a narrow, flying membrane con- 
necting the fore and hind limbs, supported by fascia articulating with the 
carpus. Neither the dentition, the skull, nor the general osteology exhibits 
any important differences. Xerus differs from Sciwrus in certain modifica- 
tions of the pelage, which is sparser and harsher, and in having shorter ears 
and tail, and in being more terrestrial in its habits. Tamas still greatly 
resembles Sciurus, but has shorter ears, internal cheek-pouches, with more 
fossorial feet, and also differs in being more terrestrial. On the other hand, 
Tamias and Spermophilus are scarcely generically separable. In Spermophi- 
dus, two premolars are constantly present, the first variable in size, but usually 
functionally developed; the dentition is stronger and the grinding-teeth are 
more firmlyimplanted. The species vary in having the ear large or rudimentary ; 
the tail long or short, terete or distichous; the skull Jong and narrow, espe- 
cially the facial portion, and the zygomatic arches moderately expanded, or 
broader and relatively shorter, with heavier and more widely expanded zygo- 
mata, etc.; the general size varies from those of the size of Tamas to those 
nearly equaling Cynomys, and in form from extreme slenderness to species 
nearly or quite as thick-set as either Cynomys or Arctomys ; yet no single sct 
