Teeth further agree with the extravascular appendages of the
skin in being shed and reproduced sometimes once, sometimes
frequently, during the life-time of an individual: the latter may
be termed ‘ interrupted’ reproduction. In some cases again, as
with certain epidermal appendages, the reproduction of the tooth
is uninterrupted, and goes on during the life-time of the individual
; new matter being added to the base as the old is worn
away from the apex, or working surface of the tooth. A tooth,
when fully formed, is subject to decay, but has no inherent power
of reparation. A tooth of limited growth can only increase in size
after its formation is completed by abnormal growth of its most
highly organised constituent, the cement. Thus, then, it appears
that, the analogy of the dental organs to those of the corneous
system holds only in their mode of development (1), in their
shedding and reproduction, and in their exposure to external
influences and to the contact of extraneous bodies: but the
antlers of Deer are similarly exposed, and are likewise shed and
reproduced annually, and also contemporaneously with the fall
of the hair; but antlers are not, therefore, classed with the
corneous tissues, any more than is the bony core of the horns of
the cavicorn Ruminants.
(1) The cells and fibres of the horny tissues are formed in and not excreted from the surface
of their formative pulps.
ODONTOGRAPHY
PART I.
DENTA L SYSTEM OF F I SHES .
C H A P T E R I.
GENERAL OBSERVATIONS ON THE TEETH OP FISHES.
If the ichthyologist have reason to complain of the monotony
[which unavoidably pervades his descriptions of the external characters
of the objects of his study,(1) the anatomist in treating of the
[dental system of fishes, finds, on the contrary, his difficulty in
[obtaining the command of language sufficiently varied to pourtray
[the singular diversity and beauty, and the interesting physiological
[relations which are manifested in that part of their organization.
[The teeth of fishes, in fact, in whatever relation they are considered,
[whether in regard to number, form, substance, structure, situation,
[or mode of attachment, offer more various and striking modifications
[than do those of any other class of animals.
‘2. Number.—If we commence with the lowest species, as the
jglutinous hag and other myxinoid fishes, we find that, the armature
[of the tongue excepted, the dental system is represented by a single
[tooth developed on the median line of the palate. In the carp,
a single median tooth above the pharynx is opposed to two dentigerous
plates below. In the Ceratodus and Ctenodus, the jaws are
formed with four teeth, two above and two below. In the chimsera,
[two lower maxillary teeth are opposed to four above. From these
fPecies may be traced every gradation in the progressive multipli-
pation of the teeth up to the pike, silurus, and other fishes in
fwhich the mouth is crowded with innumerable teeth.
3. Form.—The great variety of forms of the teeth of fishes
(!) Cuvier, Eloges, iii, p. 313.
B