324 MAMMALIA : GLIRES. XLVIII. 



FAMILY CLXXXVI. CASTORID^B. (THE BEAVERS.) 



Aquatic rodents of large size, having the molars rootless, ; 

 feet four-toed, the hind feet webbed ; body stout and heavy ; tail 

 broad, flat, and scaly; tibia and fibula distinct; no postorbital 

 process. A single genus now living, belonging to the JSTorthern 

 Hemisphere. 



552. CASTOR Linnaeus. (Lat., beaver.) 



1062. C. canadensis Kuhl. AMERICAN BEAVER. Reddish 

 brown, grayish below,. L. 40. T. 10. Weight 45 to 60 Ibs. North- 

 ern N. Am., S. to Mexico; once abundant, now being rapidly 

 exterminated. 



FAMILY CLXXXVII. SCIURID^E. (THE SQUIRRELS.) 

 Molars rooted, f:-f (upper anterior often deciduous), the last 4 

 of nearly equal size ; a distinct postorbital process of frontal bone ; 

 tibia and fibula distinct. Species of rather small size, in all parts 

 of the world except Australia. Genera about 14 ; species numerous. 

 A family which easily adapts itself to climatic and other conditions, 

 thereby forming numerous local species and varieties. 



a. Sides without membrane for " flying." 



b. Upper outline of skull nearly straight; frontal region depressed; cheek 

 pouches rudimentary; thumb with a broad flat nail; tail short, bushy; 

 ears small; fur coarse, heavy; body stout, clumsy. . ARCTOMYS, 553. 

 lib. Upper outline of skull more or less convex. 

 c. Cheek pouches present; tail moderate. 

 d. Skull comparatively thin. 



e. Thumb with rudimentary nail (other characters drawn from the 

 skull). First upper molar small, but not minute. SPERMOPHILTJS, 554. 

 ee. Thumb with well developed nail; skull narrowed anteriorly. 



/. Premolars f EUTAMIAS, 555. 



/. Premolars \ TAMIAS, 556. 



cc. Cheek pouches wanting ; tail very long and bushy, the hairs mostly on 

 its sides; skull short, broad, and rounded; first upper molar when 

 present minute, thumb nail rudimentary; eyes well developed. 



SCIURUS, 557. 



aa. Sides with a densely furred lateral membrane joining the anterior and 

 posterior limbs ; bod}' and tail depressed ; no cheek pouches ; ears large ; 

 molars subequal in size SCIUROPTERUS, 558. 



and now the commonest species, having nearly exterminated the next. (Lat., 

 the tenth.) 



M. rattus L. Black Rat. Tail not shorter than head and body ; sooty 

 black, plumbeous below ; feet brown ; introduced about 1544, but now supplanted 

 by the preceding. 



M. alexandrinus Geoffrey St. Hilaire. Roof Rat. White-bellied Rat. 

 Introduced in the Southern States. (From Alexandria in Egypt.) 



M. musculus L. Common House Mouse. Cosmopolitan ; too well known. 

 (Lat., a little mouse.) 



