plane, the laminae are seen to be developed in two distinct lateral moieties,
which become anchylosed together by means of a thin median vertical
osseous partition at their median margins; their lateral margins are
similarly anchylosed to the outer walls of the dentigerous cavity.
It is quite clear, as Cuvier observes, that the laminae are developed
successively | and that, in proportion as the anterior ones are worn
away, the posterior ones appear in readiness to replace them, so that
the due number of ridges, on the triturating surface, is always maintained
.N
evertheless, these facts will be shown to be quite insufficient
to establish the theory of dental development by transudation of
layers. Any example of a continual reproduction of teeth in vertical
succession, would be as available in that point of view ; the peculiar
application of the tooth of the Diodon to illustrate the theory of
transudation, is due merely to the form of the denticles which compose
that compound tooth.
Cuvier, in his examination of the dentition of the gymnodonts,
had employed the microscope so far as to detect the fine reticulate
impressions on one of the surfaces of the dental laminae of the
Diodon, which he rightly attributes to the impressions of vessels ; a
low power, as that of the ordinary pocket-lens, is sufficient to demonstrate
these markings. To examine the structure of the lamelliform
denticles, it is necessary to make extremely thin sections in a direction
vertical to their plane. A portion of such a section, seen by transmitted
light with a focus of half an inch, presents the appearance delineated
in Plate 39, fig. 2. Each plate here exhibits, instead of an
amorphous or sub-crystalline mass of excreted calcareous matter, a
series of extremely minute calcigerous tubes, occupying its whole
extent and having a general direction vertical to its plane. The
tubes are obviously wider at the lower side of the plate, and gradually
disappear in the clear and dense substance at the opposite surface.
When the thinnest and most transparent parts of the same section
are examined with a compound lens of f inch focus, the horizontal
partitions, which occupy the interspaces of the lamelliform teeth, are
seen to consist of a coarse cellular osseous texture, without any radiated
(Purkinjian) corpuscles, but similar to the texture of the rest of the
endo-skeleton of the Diodon. The main tubes of the dental plate are
continued immediately from the cells of the osseous septum; they
proceed for a short distance vertically, or with a slight curvature
in the substance of the dental plate, and then quickly divide an
subdivide, the branches generally coming off at an angle of 45°, being
slightly bent, crossing each other in an inextricable manner, and terminating
ultimately in the clear matrix of the upper surface of
the dEenatcahl pdleanteta. l lamella presents, at every part, the same organ.ized
structure which is above described and delineated; there is no part
which offers the crystalline characters of true or mammalian enamel.
The mucous membrane of tbe mouth and periosteum of the jaws, are
reflected into the cavities at the base of the compound tooth (PI. 38,
fig. 2 , a) ; the periosteum lines theparietes of the cavity, and the mucous
membrane forms a thick cushion, extended across its floor. From
this surface, a lamelliform pulp is developed, in which the calcifying
process takes place in a direction from above downwards: at first,
the earthy salts are deposited in the state of such minute subdivision
and in such a direction and abundance as to produce the dense an
minutely tubular structure of the dental plate ; when this has acquired
its due thickness, the rest of the pulp becomes ossified, t.e. the ca -
careous salts are deposited in less abundance, and in the panetes
and interspaces of coarse cells, instead of those of minute tubes. The
margins of the ossified pulps, by this process, become confluent with
the parietes of the general dental cavity, and the mutual adhesion
of the flattened surfaces of the impacted lamelliform teeth is promoted
by the pressure to which their exposed surfaces is subject. By the
time that ossification has begun in one pulp, a second has been
developed beneath it, and it is the portion of the pulp solidified by
the fine tubular calcification which gives rise in the macerated and
dried jaws to the loose and thin lamellae in the dental cavity. These
lamellae become fixed by means of the coarser calcification or ossification
which subsequently takes place in the remains of the pulp,
and their margins are thus anchylosed to the surrounding bone, in
a manner analogous to the fixation of the base of the ordinary
shaped teeth in other fishes.