TEETH OF FISHES. 383 



situated immediately posterior to, or on the inner margin of, the 

 sockets of the teeth in place : these foramina lead to alveoli of 

 reserve, in which the crowns of the new teeth, in diiferent stages 

 of developement, are loosely embedded. It is in this position of 

 the germs of the teeth that the Sphyraenoid fishes, both recent 

 and fossil, mainly differ, as to their dental characters, from the rest 

 of the Scomberoid family. 



It is interesting to observe that the alternate teeth are, in 

 general, contemporaneously shed : so that the maxillary armour is 

 thus preserved in an effective state. The relative position of 

 the new teeth to their predecessors, and their influence upon 

 them, resembles, in the Sphyrcena, some of the phenomena which 

 will be described in the dentition of the Crocodilian Reptiles. 

 To the Crocodiles the present voracious Fish also approximates in 



the alveolar lodgement of the teeth : but it manifests its ichthvic 



~ / 



character in the anchylosis of the fully-developed teeth to their 

 sockets, and still more strikingly in the intimate structure of the 

 teeth. 



In all Fishes the teeth are shed and renewed, not once only, as 

 in Mammals, but frequently, during the whole course of their 

 lives. The maxillary dental plates of Lepidosiren, the cylindrical 

 dental masses of the Chimceroid and Edaphodont Fishes, and the 

 rostral teeth of Pristis (if these modified dermal spines may be so 

 called), are, perhaps, the sole examples of ( permanent teeth ' to 

 be met with in the whole class. 



When the teeth are developed in alveolar cavities, they are 

 usually succeeded by others in the vertical direction, as in the 

 pharyngeal bones of the Labroids, fig. 261 : but sometimes 

 they follow one after the other, side by side, as in the Scaroids, 

 fig. 259, c. In Reptiles and Mammals the successional teeth 

 owe the origin of their matrix to the budding out from the cap- 

 sule of their predecessors of a caecal process, in which the 

 papillary rudiment of the dentinal pulp is developed; but, in 

 the great majority of Fishes, the germs of the new teeth are 

 developed, like those of the old, from the free surface of the 

 buccal membrane throughout the entire period of succession : 

 a circumstance peculiar to the present class. The Angler, the 

 Pike, and most of our common Fishes, illustrate this mode of 

 dental reproduction ; it is very conspicuous in the cartilaginous 

 Fishes, figs. 263 and 264, in which the whole phalanx of their 

 numerous teeth is ever marching slowly forwards in rotatory 

 orooress over the alveolar border of the iaw, the teeth beiii 



JL O <J O 



successively cast off as they reach the outer margin, and new 



