8 SHARKS AND RAY-FISHES. 



In all fishes tlie first step in tlie formation of tectli is the 

 simple production of a soft vascular papilla, or pimple from the 

 free surface of the membrane of the jaw near the mouth; but 

 in the Sharks and Rays these papillcE do not proceed to sink 

 into the substance of the gum, but become covered by caps 

 of an opposite free fold of this membrane. These caps do not 

 contract any organic connection with the papilliform matrix (and 

 in the torpedo they are very loose), but as this is converted into 

 dental tissue the tooth is gradually withdrawn (the points of the 

 teeth at first lying flat downward, or in the direction toward 

 the mouth,) from the extraneous protecting cap, and as they 

 become hard from being clothed with an enamelled surface, 

 they assume the upright posture on the border of the jaw. It 

 has been assumed that the number of rows of these teeth are 

 marks of the age of the Shark, and that an additional row is 

 added for each year of its growth. But this is no fiirther correct 

 than as the greater breadth of the jaw from the greater size of 

 the fish produced by longer life, affords a wider space for the 

 teeth to stand upright. A Shark of nearly full growth, if young, 

 may have no greater number of rows of teeth standing erect than 

 a couple, but there are several others at the same time in the act 

 of production ; and they are carried forward on the surface by 

 an action in the membrane itself on which they rest, until, being 

 commonly broken or worn down by the violence to which they 

 have been exposed, by the time they have reached the outer 

 edge of the jaw, an exfoliation of the membrane itself has taken 

 place, and they drop ofi" by a natural process of exfoliation, to 

 be succeeded by others, which are in their turn formed at the 

 border of the jaw nearest the mouth, and pass upward and 

 outward: the whole proceeding bearing no distant likeness to 

 that by which the nails are formed in our fingers, or hoofs in 

 the feet of beasts, to be passed onward to the part when their 

 use is required, and bv which thev are at last set free from their 

 attachment, and lost. The production and protrusion of the teeth 

 in the family of Rays is substantially the same as in Sharks; but 

 the more slender bony process that in most species projects from 

 the base is sooner broken down by the crushing process of 

 feeding on crustaceous or hard food; and the jaw is therefore, 

 in most cases, rendered almost smooth before the teeth have 

 advanced so far as to be rejected. 



There is only one other subject connected with the general 



