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TEETH OF THE VERTEBRATA. 375

I proceed next to consider the teeth of the Chiniserre {HolocephaU),
which group some authors make equal in rank with the Efasmohranchii,
which then inchide the sharks and rays only. The peculiarities of the
dental succession alone of this latter group, it appears to me, is quite
sufficient to separate them widely from all others, and it seems some-
what remarkable that this character has never been utilized by the
systematists in their schemes of classification.
The teeth of the " ratfish " {CMmcera phnnbea) are six in number, of
which two belong to the lower and four to the upper jaw. The two
inferior ones may be described as broad, slightly-curved plates of mod-
erate thickness in the form of a right-angled triangle. That border
which corresponds to the perpendicular is almost straight, and is lodged
in a shallow groove which runs lengthwise along the inner surface of
the jaw ; that which represents the base is applied to the corresponding
surface of the opposite tooth ; while the border representing the hypoth-
enuse forms the free cutting edge of the tooth. This border is some-
what devious, being interrupted by three prominences. The niner
surface is also slightly ribbed. The two posterior upper teeth are
similar plates of a quadrilateral form with their free edges roughly
serrat^e. The two anterior teeth above somewhat resemble ordinary
mammalian incisors, and are large and scalpriform. This peculiarity
has given them the name " rabbit-fish " or " ratfish." Each tooth
has a cavity in the edge by which it is attached and in which the pulp
is lodged. But a single set of teeth are developed during the life of the
individual, and these are of persistent growth. Another living allied
genus, Callorhi/nchus, is found in Australian seas, in which the teeth are
similar to those of Chimaera, but in the two fossil genera, Edaphadon
and Passalodon, supposed to belong to this group, the teeth are anky-
losed to the jaw, which is more or less bony. On this account it is
more than probable that they are to be referred to the dipnoans rather
than to the chimaeroids.
The Teeth in True FisJies.—The teeth of the class Pisces, although
apparently presenting an extensive range of modification, have not,
debarring the dipnoan ganoids and the plectognath teleosts, as a general
rule, departed very widely from the simple conical pattern. There are
some forms, however, in which the structure and arrangement are quite
anomalous. It is in this group that the maximum development, as far
as numbers is concerned, is reached. The salmon, pike, and some per-
coids may be cited in which teeth are developed in almost every con-
ceivable part of the mouth and number many thousands ; while in
others, as the carps and suckers, they are few and confined to the
pharyngeal bones. In others, again, as the pipefishes and sea-horses,
teeth are entirely absent.
The teeth of the dipnoans are unique among fishes, and, like those of
the chimperoids, are limited in number and grow from persistent pulps.
The teeth of the dipnoans, the dental plates of the chima^roids, and the
"
so-called " rostral teeth of the sawfish are the only examples so far
known of permanent teeth to be met with among piscine forms.
The dental armature of Cemtodus Fosteri (Fig. J 94), which may be
taken as illustrative of this peculiar group, has six teeth, of which four