318 APPENDICES. 



for the building of their habitations. These important organs 

 contribute, therefore, in an especial manner, to supply them both 

 with food and shelter. 



The incisor teeth of the beavers are two in number in each 

 jaw ; they are broad, flat, and generally colored of a deep orange 

 or almost chestnut brown anteriorly, and pass into acute angles 

 on their posterior surface. Their extremities terminate externally 

 in a cutting edge, and shelve considerably inward ; for the ante- 

 rior surface being alone coated with enamel, and consequently 

 oifering the greatest resistance, is less easily worn down by the 

 action to which they are exposed. Those of either jaw cor- 

 respond exactly with their opposites, and the form of the articu- 

 lation of the lower jaw admitting of little or no lateral motion, 

 their action is always from behind forward and vice verna. They 

 have no true roots, but arc of equal thickness throughout, and 

 are implanted within the jaw in sacs or capsules, which repro- 

 duce them from the base as fast as they are worn down at the 

 extremity. So strong a tendency have they to increase by this 

 process, that whenever one of the incisors of either jaw has been 

 accidentally injured or destroyed, the opposite tooth, meeting 

 with no resistance from its antagonist, is propelled forward by a 

 continual enlargement from the base to such an extent as to be- 

 come at length perfectly monstrous. This mode of growth is 

 common to the whole order, and the number of the incisor teeth 

 is also the same in all the groups that compose it, with the excep- 

 tion of the family of which the hare forms the type. 



The entire absence of canine teeth, leaving a vacant space of 

 some extent between the incisors and the molars, is another char- 

 acter which the beavers have in common with all the Rodent an- 

 imals ; but the structure of their molar teeth differs from that of 

 any other group. These latter organs furnish indeed the best 

 characters that have yet been employed for the separation of the 

 Rongeurs into distinct and natural genera. In the beavers they 

 ai'e four on each side in either jaw, and their crowns present a 

 flattened surface on which the lines of enamel are so disposed 

 as to form three folds on the outer side and one on the inner in 

 those of the upper jaw, while those of the lower offer an arrange- 

 ment directly the reverse. They were formerly suspected by M. 

 F. Cuvier, who has paid particular attention to the teeth of the 

 mammiferous quadrupeds, to be destitute of proper roots, and to 



