322 MAMMALIA: GLIRES, — XLVIII. 
Famity CLXXXVI. CASTORIDAL. (THE BEAVERS.) 
Aquatic rodents of large size, having the molars rootless, 4-4; 
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 species now living, belonging to the northern 
hemisphere. 
549. CASTOR Linneus. (Hat., the Beaver.) 
1052. C. fiber L. Braver. Reddish brown, grayish below. 
L. 40. T. 10. Weight 45 to 60 lbs. Northern regions, S. to 
Mexico; once abundant, now being rapidly exterminated. (Eu.) 
(Lat., the beaver or badger.) 
Famity CLXXXVII. SCIURIDAG. (TuHe SquirReELs.) 
Molars rooted, 3-3 (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 12, species 150. The 
variations in color, etc., are extremely great, and the number of 
well defined species is very much less than was once supposed. 
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. ARcToMmys, 550. 
bb. Upper outline of skull more or less convex. 
c. Cheek pouches present; tail moderate. 
d. Skull strong and massive; ears rudimentary; thumb with well de- 
veloped nail; body heavy, thickset. . . . . . CyNomys, 551. 
dd. Skull comparatively thin. 
e. Thumb with rudimentary nail; (other characters drawn from the 
Skull). 6 cee we? a A. 4) SP ROIs 
ee. Thumb with well developed nail; skull narrowed anteriorly. 
TAMIAS, 553. 
ec. Cheek pouches wanting; tail very long and bushy, the hairs mostly 
on its sides; skull short, broad and rounded; thumb nail rudimen- 
tarv; eyes well developed. . .. . =. . . + Scrurus, 554. 
aa. Sides with a densely furred lateral membrane joining the anterior and 
posterior limbs: body and tail depressed; no cheek-pouches; ears large; 
molars subequal in size. . . . . « ». + »« SCIUROPTERUS, 555. 
with stiff hairs; cosmopolitan ; introduced into America about 1775, 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 Geoffroy 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.) 
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