SCI URIDiE— GENERAL CONSIDERATIONS. G39 



mophilus to Cynomys is almost again without hiatus. Arctomys is again not 

 greatly different from Cynomys and some of the larger species of Spe?-/nophi- 

 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 Sciurina, or "True Squirrels", 

 and the latter (with its subdivisions) with Cynomys and Arctomys to form 

 the subfamily, or "tribe", ArctomyincB, or "Burrowing Squirrels". I find, 

 however, that here no line of separation can be drawn, Tamias and Spermo- 

 vliilus so thoroughly intergrading that no feature serves to trenchantly separate 

 them. A much better hiatus occurs between Cynomys and Spennophilus, 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 Sciurus 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. Tamias 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 Spennophilus are scarcely generically separable. In Spcrmophi- 

 lus, two premolars are constantly present, the first variable in size, but usually 

 functionally developed ; the dentition is stronger and the grinding-teeth are 

 more firmly implanted. The species vary in having the ear large or rudimentary; 

 the tail long or short, terete or distichous ; the skull long 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 Tamias 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 set 



