1896. 



THE TEETH OF FISHES. 3^7 



In teleostean fishes there are found a number of anomalous, 

 composite tooth-structures, which have resulted from a modification 

 of continuous succession. In the porcupine-, or globe-fish (Diodon), 

 the upper and lower jaws are each provided with two complex 

 structures of a dental nature. The one of these (Fig. 14, a) situated 

 along the anterior border of the jaw, constitutes a blunt cutting edge, 

 while the other (b), which is larger in size and situated farther back, 

 presents an oval crushing surface. A section through the jaw shows 

 that the dental organs consist of a great number of superposed plates 

 of dentine, free when young and widely separated, but firmly and 

 closely amalgamated by a bony cement when they approach the 

 surface of the jaw to replace their predecessors which have been worn 

 away. The obUquity of the grinding surface to the plane of the plates 

 causes portions of several of these to be exposed at a time. In the 

 parrot-fish (Scams) the teeth are developed in great profusion in the 

 interior of a large, common alveolar cavity in the bone of the jaw. 

 The younger teeth (Fig. 15, b) are free, that is to say, they are held 

 together only by the organic materials occupying the alveolar cavity ; 

 but the six or eight oldest teeth (a) of each vertical series are firmly 

 bound together by a bony deposit. The teeth lie on their sides, and 

 are piled up one above the other with their apices directed outwards, 

 so that the biting surface is made up of the sides of the amalgamated 

 teeth (Boas, 2). 



Arrangement of Teeth in the Different Groups of Fishes. — 

 In the lamprey [Petromyzon) the teeth are uniformly and symmetrically 

 disposed over the surface of the suctorial disc. The teeth are mostly 

 conical in form, but those in the centre of the mouth are larger and of 

 a more complex shape (Fig. 16). In the hag-fish (Myxine) there are 

 several sharp teeth arranged in four rows on the extremity of the 

 lingual cartilage, but only a single tooth in the roof of the mouth. 



Although in most sharks the largest teeth are to be found at the 

 front of the jaw, in the Port Jackson shark (Cestracion) (Fig. 10) the 

 front teeth are small and pointed; whereas the hinder teeth have the 

 form of broad crushing plates, adapted for breaking the shells of the 

 molluscs that constitute the food of this shark. Numerous examples 

 of such a dentition are found in extinct sharks, e.g., Acrodus, Orodtis, 

 etc. In Pletiroplax and Pcecilodus the crushing teeth tend to fuse into 

 continuous plates, a form of coalescence which finds its culmination 

 in Cochliodus. The individual crushing teeth are indistinguishable in 

 CocJiliodus, but each ramus of the jaw is provided with two spirally- 

 coiled dental plates (Fig. 11, a and b). There is nothing equivalent 

 to the shedding of teeth, but the disused external portion of the dental 

 plate (Fig. 12, e) becomes buried in the edge of the jaw as the spiral 

 revolves slowly outwards. In the skates and rays the dentigerous 

 surface of the jaw is much more rounded than in the sharks, more 

 rows of teeth are in use at a time, and the teeth are more closely set, 

 with smaller intervals. In some forms the teeth are so blunt and so 



