324 MAMMALIA: GLIRES. — XLVIII. 
Famiry CLKXXVI. CASTORIDAL. (Tue 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 genus now living, belonging to the Northern 
Hemisphere. 
552. CASTOR Linnzus. (Lat., beaver.) 
1062. C. canadensis Kuhl. American Braver. Reddish 
brown, grayish below. L. 40. T. 10. Weight 45 to 601bs. North- 
ern N. Am., S. to Mexico; once abundant, now being rapidly 
exterminated. 
Famity CLXXXVU. SCIURIDAS. (THE Squrrrets.) 
Molars rooted, :§ (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. 
uw. 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. 
bb. Upper outline of skull more or less convex. 
v. 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. SpERMOPHILUS, 554. 
ee. Thumb with well developed nail; skull narrowed anteriorly. 
Jj. Premolars?, . 2 + 2 6 1 ew © | 6. Evramias, 555, 
ff. Premolars} . «2 se eee : . Tamas, 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. 
Scrurvs, 557. 
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. . . . . . . . » SciurRopreRvs, 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 Geoffroy St. Hilaire. Roof Rat. White-bellied Rat. 
Introduced in the Southern States. (From Alexandria in Egypt.) 
M, musculus Y.. Common House Mouse, Cosmopolitan; too well known. 
(Lat., a little mouse.) 
