FISHES. 355 



the same sort of mechanical process, and it falls with the tooth which 

 it supports. 



The renewal of the teeth continues during a considerable portion of 

 life, and according to all appearance, there is tooth for tooth, which 

 come at very uncertain periods, as happens in the leaves of green 

 trees. 



The new tooth sometimes grows beneath, sometimes at the side, or 

 behind or before the old tooth. 



In the replacement, which takes place vertically, and which parti- 

 cularly takes place in the round teeth, when the ossified nucleus of 

 the old tooth is united to the jaw, it is nourished by it, and its texture 

 becomes cellular ; its cavity is filled, and when the crown is detached, 

 the surface of the bone is continued ; but deeper still is a cavity, where 

 the tooth that is to replace the old one begins to be formed : it pene- 

 trates at the proper period the surface of the bone, and undergoes the 

 same changes as its predecessor. 



The renewal of a bone by the side is peculiar to the great conical 

 or hooked teeth and also the trenchant ones, the new tooth pierces 

 the bone on the side of the old tooth, but the latter does not fall the 

 less by rupture as usually takes place. 



Amongst the singularities connected with the dentition of fishes, 

 we may set down those of the pharynx in cyprins, in the jaws of the 

 scarus, and in those of the tretrodons and diodons. 



The cyprins have no teeth, except in their inferior pharyngeans, 

 which surround the sides of the pharynx like half collars ; these teeth 

 are few but very strong ; on the superior surface there is a triangular 

 plate of dentary substance or enamel, but very hard, and is com- 

 monly called carp stone ; it is enchased, and looks as if it 

 were set in a dilatation of the basilar bone. It grows by layers 

 formed on the surface that touches the bone, and it is against that, 

 that the inferior pharyngeans compress and bruise the food. 



The jaws of the scarus have nearly the external form of a parrot's 

 beak ; there are little holes at their base by which the teeth, the 

 germs of which may be seen in the interior, can come out to fix 

 themselves on their surface, on which the preceeding teeth already 

 had formed small warts in quincunx ; (a) they are carried also very 

 gradually towards the edge where they become endowed with 

 activity, and then those which have preceded them in this situation 

 fall in consequence of detrition. 



Upon the pharyngeans of these fishes the teeth are trenchant, and 

 grow vertically and in a quincunx on the surface of the bone ; so that 

 according as the old ones are worn the new ones come up behind 

 them. 



The same process occurs in the pharyngeal! teeth of labrus, with 

 this single difference that they are round instead of being trenchant. 



{£$> (a) Quincunx is a term borrowed from gardners, and which is thus explained. 

 One row of plants is placed sixteen inches from the end of the plot and sixteen 

 inches apart, a second row is placed twenty-two inches and a half from the end and 

 fifteen inches apart, the third row is placed fifteen inches from the end and fifteen 

 inches apart, and the fourth row again is twenty-two inches from the end and fifteen 

 inches apart. The plants then will form rhomboidal squares. — Exg. Ed. 



A A 2 



