8 
SHARKS AND BAY-FISHES. 
In all fishes the first step in the formation of teeth is the 
simple production of a soft vascular papilla, or pimple from tne 
free surface of the membrane of. the jaw near the mouth; but 
in the Sharks and Hays these papillte 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dental tissue the tooth is gradually withdrawn (the points of the 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 further 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 off 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 by which they 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 
