ANATOMY OF THE BEAVER. 65 



when once fully formed, is unchangeable, and the root 

 serves merely as a support for the crown. The beaver 

 has 20 teeth, viz., 2 incisors and 8 molars in each 

 jaw. The anterior molars, 4 in all, are deciduous; 

 the crowns of these teeth resemble the permanent 

 ones ; the upper have three divergent roots and the 

 lower two. They are gradually protruded from their 

 sockets by the permanent teeth rising beneath them. 

 Whether the cutting teeth of the beaver should be 

 regarded as canine teeth rather than as incisors, has 

 been questioned, inasmuch as they extend back into 

 the superior maxillary bone. It is generally held that 

 this relation is only to accommodate their great 

 length, and that their uses and connection with the 

 intermaxillaries are sufficient to sustain the ordinary 

 view. The incisors of the beaver are nearly triangu- 

 lar, and extend far into the jaw, with a circular 

 curve, the upper forming more, and the lower less 

 than half the circumference of a circle, the radius of 

 the curve in the upper being one inch, in the lower 

 1 " 75'". They are composed chiefly of dentine, having 

 a thin layer of orange-colored enamel on their ante- 

 rior surface and angles. The upper incisors are con- 

 tained in a sheath which projects into the nasal cav- 

 ity, the end of the tooth being separated by a thin 

 vertical plate of bone from the first molar. The 

 lower incisors pass under the roots of the molars to a 

 point behind them and below the posterior foramina. 

 The dentine of the incisors, being softer than the 

 enamel, wears away and gives to the end of the tooth 

 a beveled or chisel form, with a sharp anterior edge 

 of enamel, so that they are called scalpriform teeth. 

 The portion of the tooth inclosed in the socket has 



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