24 Proceedings of the Royal Physical Society. 
cessarj, and this is supplied by the broad flat horizontal tail, 
which is constructed on the best principles for attaining such 
an object. Were it different or differently placed, the end would 
not be answered ; suppose it vertical like a fish's tail, it would 
answer equally well as a swimming organ, but not as a 
counterpoise. It must be powerful, and it must be hori- 
zontal, so as to press broadly downwards, and as the pur- 
pose here is to increase resistance and friction, and not to di- 
minish it, it is denuded of hair, or nearly so, and covered 
with polygonal scales. A few scattered hairs occur, inter- 
spersed between them, but these are not abraded as they 
would have been had the tail been used as a trowel. That 
this is the interpretation of the structure and purpose of the 
tail is I think self-evident from its fitness. The habit of 
flapping the tail on the ground before plunging into the water 
is probably only the mechanical repetition of the action with 
which it habitually starts into motion, and which in the water 
is essential to its progress. 
The teeth of the beaver are often quoted as good examples 
of the mode in which rodent teeth grow from the pulp at their 
base, with a hard enamel-like steel on the outer edge, and 
softer material on the inner side, and thus have their sharp- 
ness and chisel -like form always kept up by the very thing 
which at first sight would seem to be likely to make them 
blunt — viz., their constant use. The incisor teeth in the 
foetus are conical, thus showing that the chisel form in the 
adult is the result of abrasion. The specimens sent me are 
from the neighbourhood of Moose Factory. 
I have adopted the specific name Americanus given to this 
species by the Russian naturalist Brandt, who has separated 
the American animal from the European and Asiatic (the true 
Castor fiber) on osteological grounds, chiefly drawn from the 
skull. For the reason alluded to above (want of specimens 
for comparison), I can give no opinion as to the propriety of 
this separation. 
Mus leucopus, Rafin. — In his description of this species, 
Sir John Richardson says, — " The tail is thickly clothed with 
short hairs, lying pretty smoothly, no scales whatever being 
