356 FISHES. 



The jaws of the tetrodons would very nearly resemble the pharyn- 

 geal! bones of scarus, were it not that each of these teeth, or rather 

 of these lamina, occupies the whole breadth of the bone. The poste- 

 riors are the most recent and the anteriors the most worn. 



In the diadons there are two series of lamina, the one forming the 

 margin of the jaw, the other series forming a disk situated more 

 behind, and separated from the margin by a very shallow depression; 

 they succeed each other, and in s\ich a way, as that in the disk the 

 posteriors are always the most recent, and in the edge the superior. 



But the common character applicable to these two forms of denti- 

 tion, is that the entire jaw is only armed by two or even a single 

 compound tooth, the lamina of which grow by the transudation of the 

 pulpy lamina, which are interposed between them, and are then 

 united by the same mass of enamel. 



The chimera has compound teeth like the two genera we have 

 spoken of, but they are generated and grow on the germs, like 

 threads instead of lamina, and their interior tissue is pierced with 

 fine tubes like a rush, or the teeth of an orycteropus. There are 

 four plates in the superior jaw and two in the inferior. 



The flat and broad teeth of mylobates (fishes belonging to the 

 family of the rays) are also compound, so far, at least, as their sub- 

 stance is formed on a vast number of pulpy filaments, and is invested 

 with the enamel common to the whole. 



The teeth of the lamprey arc thin horns, moulded on the fleshy 

 germs ; and there are various forms and directions on the lips, on the 

 jaws, and on the tongue ; to these we shall afterwards return. 



In the squalus with trenchant teeth the nucleus of the tooth is per- 

 manently cartilaginous, and is for a long time flexible, so that the 

 new teeth, which always grow up behind the old ones, remain in the 

 species where they are trenchant, as in the white sharks, in layers 

 behind, and sometimes even superposed upon one another in several 

 ranges. They rise up and the base of their nucleus assumes a con- 

 sistence when the time has arrived at which they must become active. 



There are some species of squalus in which a portion of the teeth 

 is flat, and broad, and compound, like those of mylobites. 



In particular articles we shall enter into the detail which may be 

 necessary for giving a description of the dentition of each fish. 



Deglutition. 



In the greater part of osseous fishes, — besides the lips, Avhich even 

 when they are fleshy and have no muscles proper to themselves, 

 must of necessity be without the power of retaining the food in the 

 mouth, — we find generally within each jaw, behind the anterior 

 teeth, a sort of membranous veil, or little valve formed by a fold of 

 the internal skin and directed posteriorly, the effect of which must be 

 to impede the food, and especially the water that is swallowed for 

 respiration, from running back by the mouth. 



The food, seized by the teeth of the jaws, retained by this little 

 valve, carried further behind by the teeth of the palate and the 

 tongue when they exist, is prevented by the dentations of the branchial 



