CHARACTERISTICS AND HABITAT. 29 



its down motion are several times stronger than for 

 either its upward or lateral movements. He is able 

 to turn his tail under him and sit upon it, or to use 

 it extended behind him as a prop while sitting up 

 upon his hind feet. Young beavers, while feeding 

 or resting, usually swing their tails around by their 

 side in the same manner as a cat, but with the lower 

 surface uppermost. It has often been asserted that 

 the beaver uses his tail as a trowel in preparing mor- 

 tar from mud. This mistake is sufficiently explained 

 by stating that he uses mud and soft earth, sometimes 

 intermixed with roots and grass, precisely as he finds 

 them, and without any preparation whatever, for 

 their conversion into mortar. But he uses his tail to 

 pack and compress mud and earth while constructing 

 a lodge or dam, which he effects by heavy and re- 

 peated down strokes. It performs in this respect a 

 most important office, and one not unlike some of the 

 uses of the trowel. 



The eye of the beaver is disproportionately small, 

 the optic nerve a mere thread, and its foramen one 

 of the smallest in the skull. As his vision is of short 

 range, he does not rely upon this sense except with 

 reference to near objects. On the contrary, his hear- 

 ing is very acute. The auditory tube, which is usu- 

 ally about half an inch in length, terminates in a 

 tympanic cavity, or bulla, of nearly globular form, 

 and large relatively to the size of the skull. It is 

 considerably larger than in man, and its size is, to 

 some extent, the measure of the strength of this 

 sense. This provision to intensify the hearing is, 

 however, equally conspicuous among the carnivora. 

 Upon this sense the beaver relies to a much greater 



