THE TEETH OF FISHES. 233 



down much faster, so that the edge is always formed by a 

 prominent sharp tooth, which, as the wearing down of the 

 bone progresses, falls off, and the next one beneath it comes 

 into play. The arrangement recalls the way in which a 

 scythe or a chisel is assisted in keeping its edge by being 

 made of a plate of steel welded between two plates of 

 softer iron. 



The pharyngeal bones are also remarkable ; the two lower 

 are united into one, and the stout bone so formed is armed 

 with teeth ; it is antagonised by two upper pharyngeal bones 

 similarly armed. It carries teeth which are anchylosed to 

 it, and which are so disposed as to keep the surface con- 

 stantly rough. When they are freshly formed the teeth 



c 



have flattened thin edges, something like human incisors. 

 The teeth are coated with enamel, and thus, when calcifi- 

 cation has proceeded so far as to obliterate their central 

 pulp cavities, after the tooth is worn to a certain point 

 (c in Fig. 99) it presents a ring of enamel, inside which 

 comes a ring of dentine, and inside this a core of secondary 

 dentine, as seen in the figure. Owing to the different hard- 



( l ) Lower pharyngeal bone of Pseudoscarus. a. Posterior border, at 

 which the teeth are unworn, c. Oval areas formed by teeth, the points 

 of which are worn off. b. Anterior edge of bone, at which the teeth are 

 almost completely worn away. 



