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372 — DENTAL ANATOMY.

the blue shark (Carcharias). These lateral serratures may become so
strongly developed as to give to the tooth a distinct comb-like appear-
ance e.g. lower teeth oi' Xotidanus (Fig. 191).
Fig. 191.

Teeth of Nolidanus (after Gunther).
The most remarkable modification in the dental organs of sharks is
exemplified by the Port Jackson shark (Cestradon), in which the poste-
rior teeth gradually become broad and form a regular pavement on the
surface of the jaws similar to that seen in many rays. This structure
exists in consonance with the shellfish-feeding habits of the animal, in
the exercise of which great crushing and comminuting power is required
to. be exerted. These fishes are of especial interest, inasmuch as they are
the only living representatives of an extensive and widely-distributed
group which appeared on the earth far back in the Devonian Epoch,
and whose remains, as Owen justly remarks, " would have been scarcely
intelligible to us unless the key to their nature had been afforded by the
teeth and spines of the living cestracionts."
The teeth of the anterior part of the jaw (Fig. 192) are the smallest,
and present a compressed conical form with the apex produced into a
sharp point. Proceeding backward, they gradually assume an oblong
oval outline, progressively increasing in size, their sides becoming
applied to each other in such a manner as to form a regular pavement.
The maximum size is attained at about the fourth tooth from the poste-
rior end of the series, after which they decrease rapidly, although still
preserving their modified crushing form.
The progressive changes in size and form, as well as the disposition,
of the most highly modified teeth in this animal, are seen to be in direct
accord with the uses to which they are put, and serve to illustrate, as so