24 Proceedings of the Royal Physical Society. 



cessary, 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 



