64 THE AMERICAN BEAVER. 



their correspondence with other structures of the 

 body, the comparative anatomist is able to determine, 

 by an inspection of these organs alone, the kind of 

 animal to which they belong. The rodents gener- 

 ally derive their food from the vegetable kingdom. 



Before describing the teeth of the beaver, we may 

 premise for the general reader a few facts in relation 

 to the dental organs. Mammalian teeth are composed 

 of substances essentially resembling bone, of which 

 three kinds are usually present, viz.: the external 

 hard covering or enamel; dentine, which forms the 

 body of the tooth; and cementum, or crusta petrosa, 

 which is deposited on the surface, and usually on the 

 dentine of the root. The divisions of a tooth are the 

 crown, or portion above the gum; the root, or part 

 inclosed in the socket; and the neck, or point of 

 junction between the crown and the root. There are 

 three kinds of teeth: the front, or incisive; the back, 

 or molar; and the canine, or intermediate teeth, whose 

 development is a striking feature in the jaws of the 

 Carnivora. These are wanting in the Rodentia, and 

 in the Edentata the incisive teeth are wanting. Some 

 teeth are permanent, while others are deciduous, the 

 so-called milk teeth, whose places are supplied by 

 those of the permanent class. In some cases, teeth, 

 when once formed, are unchangeable in their develop- 

 ment or growth, and are therefore called "rooted" 

 teeth. In other instances the teeth are so constituted 

 that they grow continually as they are worn by use, 

 and are called "rootless" teeth. Rootless teeth are 

 generally cylindric or prismatic, with an expanded 

 open cavity, containing a pulp organ capable of sup- 

 plying an unlimited growth, while the rooted tooth, 



