RODENTIA—CASTORINAE—CASTOR CANADENSIS. 357 
rare. The lightest specimens I have seen are 1548 from the upper Missouri, and 1337 from 
the Colorado river of California. 
Measurements of No. 898. 
Inches. Lines. 
UBER OID item eee tanta ao ae acc cacaes oe  sescecnass, 
Eye Be bp tense ae ee enc Wee 1 3 
GP ot noone Seis oe ose eee see 2 3 
Toot Otifail . Sows saesae Seen oe 8 6 
Real yi oOMion ss sent eee Sa Se 10 6 
end of outstretched hind legs.....------- i ee 
Tail, from root to end of vertebree_-.-....------ + 4 
from scaly portion to end of vertebre-.----- Bie [ese he 
RL Gt Se tere em ae re a ore 1 2 
Hargs-heipht,) posteriorly sa. 22s case cosene eee Slo~-oaaeecs 54 
shevemotich s25 seo sea5* SOF Lk Sebati ee 7 
internally above skull___..--------|---------- 4} 
Arm, fore foot to end of claws--.-------------- 1 33 
Lord TS eso Soe See nS CARE ee Pee Saal} seas 34 
Leg, from knee joint to end of claws__---------- 4 6 
hind foot, from heel to end of claws.------- 2 8 
TOTP ESE CME Wi ata so pe A a i re i i 4 
In young specimens the tibia and fibula are distinct but anchylose with age. The line of junction is, however, quite 
distinct, even in old specimens. 
Skull.—The skull of the beaver is very massive and heavy, and has many peculiarities not 
shared by other genera of rodents. The nasal bones are broad and short, widest anteriorly and 
narrowing posteriorly ; the external border a convex curve. The line drawn across the pos- 
terior extremity of these bones falls either on the small tuberosity of the ante-orbital process of 
the frontal or a little behind it, in no case reaching as far as the middle of the orbit, as in the 
European beaver. The nasal processes of the intermaxillary do not extend quite so far back, 
but attain a point between the tuberosities referred to. There is no post-orbital process to the 
frontal bone, although a tubercle not quite so prominent as that in the anterior process may, 
perhaps, be considered to represent one. The malar bone is much as in the Sciurinae, except 
that it is proportionally much higher in its middle portion. The anterior border of the zygo- 
matic arch is not formed by the malar bone, but by the maxillary which shows a thin edge 
applied against the malar for half its extent exteriorly. The malar extends up to the anterior 
end of the orbit, which is there constituted by it, and is separated from contact with the frontal 
by the small subquadrate external face of the lachrymal. The malar sends up a broad flattened 
angular process which constitutes the postero-inferior wall of the orbit. Posteriorly it passes 
along the under surface of the malar process of the temporal, and passes a little behind this 
process. It also forms the exterior wall of the glenoid cavity. 
The incisive foramina are long and narrow, situated about midway between the incisors and 
molars, occupying rather less than one-third of this interval. They are situated almost entirely 
in the intermaxillary bones ; the posterior ends, with a thin vertical plate, (extending forward 
for half their length,) belonging to the maxillary. The palatine bones extend forward in an 
