66 THE AMERICAN BEAVER. 



a conical cavity, filled with the pulp organ, which 

 forms successive layers of dentine so that the tooth 

 continues to grow as fast as it is worn away. As it 

 sometimes happens that a tooth of this kind is broken 

 off, the opposite tooth has been found to grow until 

 its outward projection constituted nearly a circle. 

 The incisors, it need hardly be said, are, according to 

 the definition, "rootless" teeth. The molars are 

 firmly and compactly set in the jaws. The upper 

 set are supported on their outer edge by a firm alve- 

 olar ridge, but on the inside their sockets are shallow. 

 The lower set are more deeply and strongly implanted 

 in the jaw. The first molars are largest and longest, 



and the last are the 

 ^i<^- ^- smallest, and project 



but little from the jaw. 



The inner surface of 



the upper molars has 



Left upper molar, Left under molar, , , . ,. , 



outside. outside. ^uc deep longitudmal 



groove extending to 

 the end of the tooth, and the outer surface three 

 grooves. These are similar, but reversed in the lower 

 tooth. The surface of the crown is marked by a 

 complicated folding of enamel, of which a diagram is 

 given (Fig. 3). 



The dentine between the layers of enamel is worn 

 so as to leave the latter in ridges. Each molar is 

 curved so as to present two concave surfaces. The 

 upper set curve backward and outward; the lower set 

 forward and inward. The surface line of the upper 

 set is slightly convex, that of the lower is concave. 

 Their surfaces are thus brought into apposition, and 

 the bearing of the teeth in the sockets is effected 



