THE TEETH OF FISHES. 231 



The new teeth or plates of dentine thus formed at the 

 base of the hemispherical masses within the jaws (at the 

 point a), or low down in the substance of the jaw, do not 



FIG. 99 (') 



corne into use by the ordinary process of displacing their 

 predecessors, and being in turn themselves replaced, but 

 fresh plates only come into use by the actual wearing away 

 of all that is above them, both dentine and bone, so that 

 they come to be the topmost portion of the jaw. The 

 margins of the jaw are, however, mainly built up of dental 

 tissues, there being but little bone in their interspaces. 



Tetrodon has not the rounded triturating disk of the 

 Diodon, or has it but feebly represented ; and the margins 

 of the jaws are sharper. 



In the Parrot-fishes (Scarus), which are not very nearly 

 allied to the Gymnodonts, somewhat similar beaks are found, 



( ] ) Jaws of the Diodon. a. Base of the dental plates, where new lamella? 

 of dentine are being developed, b. Margin of jaw, formed mainly by the 

 sides of the denticles, c. Compound tooth, made up of the superimposed 

 lamella) of dentine anchylosed together. 



