The head is relatively broad, massive, compressed laterally, and 
noticeable for the flatness of its upper surface which continues to 
the shoulders without a depression of the nape, and with scarcely any 
constriction of the sides of the neck behind the ears. The peculiar 
shape of the head as a whole may be correlated with the preponderance 
of right angles and planes which the skull exhibits (see frontispiece). 
The undersurface of its retreating chin is nearly flat, with the 
distal end broadly convex. The muzzle is blunt and hairy except for 
the margin of the external nares and the cleft of the upper lip. The 
opening of the mouth looks contracted from the hairy thick lips which 
partially expose the incisors. Cheek pouches are absent. The long, 
stiff, and numerous vibrissae are mostly colorless. Some of the long- 
est ones, when laid backwards, may reach beyond the shoulders. 
The diminutive eyes (diameter between the canthi, about 1/4 inch), 
situated about halfway in a line between the extremity of the snout and 
the middle of the ear, appear sunken and inexpressive. 
The ears of moderate size, rounded above, lobate below with an 
antigral fold, and resembling the human ear in form, are uniformly 
furred with soft hairs. They are situated upon the most prominent pos- 
tero~superior aspect of the head rising about as high as the fur 
surrounding them, | 
The fore and hind pentadactyl limbs are short and approximately 
equal, The palm of the forefoot has five tubercles and the sole of the 
hind foot has six. The forelimbs are close to the head. The back of 
the forefoot has uniformly short adpressed hairs like those on the 
concavity of the ear. The smooth palm of the hand and the undersides 
of the digits are naked and pinkish. The functional and opposed thumb, 
indicating true prehensile powers of the hand, bears both a small, 
stumpy, compressed obtuse claw, and a true claw. The remaining digits 
are long and well formed, The third digit is the longest; the second 
and fourth subequal, and the fifth is the shortest with the tip of this 
claw being a little longer than the base of the fourth digit claw. The 
compressed and slightly curved claws are nearly as long as their 
respective digits. The nonwebbed hind foot is a little longer than the 
forefoot including the claws. The heel is naked and flat. The digits 
and claws are like those of the forefoot except the fore claws are 
much longer and stouter, adapted for digging. 
The distinctive feature of the external anatomy of the mountain 
beaver is its cylindrical tail, a mere stump, about 1/4 inch shorter 
than the forefoot. It is furred like the body. Though vestigial, it 
is not concealed as is the vestigial tail of certain other rodent 
species. 
