250 FISHES CHAP. 
like in many other respects, also has teeth implanted basally in 
the gums, and quite free from any special connexion with the 
jaw-bones. In some Teleosts with movable teeth, the latter 
are merely attached to the jaws by fibrous, and often elastic, 
ligaments, as in the Pike (#sox) and the Angler-Fish (Lophius). 
As a rule, however, the teeth are directly ankylosed to the 
bones developed in relation with the jaws. Very rarely, as, for 
example, in some Characinidae, are the teeth implanted in 
sockets. 
Nearly all Fishes are polyphyodont, that is, the old teeth are 
constantly replaced by new teeth as fast as they become worn 
down or fall out. In the Sharks and Dog-Fishes, for example, 
where the teeth are arranged in rows parallel to the axis of each 
jaw, the functional teeth along the upper edge of the jaw are 
usually erect, while those in the 
rows more internally situated point 
inwards towards the oral cavity ; 
and behind these again there are 
rows of developing teeth in different 
stages of growth, and partially hidden 
beneath a projecting fold of the oral 
mucous membrane (Fig. 152). As 
the teeth in use become lost they 
are successively replaced by the 
inner rows, which, with the mucous 
Fic. 152.—Transverse section through membrane in which they are em- 
the jaw of a Shark (Carcharias), | __- % 
showing how the teeth are re. bedded, move forwards to: thevedze 
placed. ¢, Cartilage of the jaw; of the jaw, where they become erect 
t, functional tooth; ¢’, its im- _ 
mediate suecessar; 2”, ¢”, stil and functional. The teethioieiae 
pe eee tse ne es Holocephali and of the Dipnoi are 
(From Ridewood.) not shed, but the loss which they 
sustain through wear and tear is 
made good by persistent growth at their bases. In the Teleo- 
stomi the succession is less regular, new teeth being formed between 
or at the bases of the old teeth. In the case of socketed teeth 
the succession is usually vertical, the new teeth being formed at the 
sides of the old ones; and by the absorption of the bases of the 
latter, the former come to lie directly below them, and eventually 
they occupy the same sockets. 
As might be expected from the remarkable diversity in the 
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