The structure and formation of the compound tooth of the
Diodon, illustrate the nature and causes of the apparently lamellar
structure of the teeth of certain mammalia, especially of the conical
tusks, the growth of which is uninterrupted. The polished surface of
a vertical longitudinal section of one of these teeth exhibits concentric
lines which run parallel with the outer contour of the section ;
these teeth, moreover, are commonly resolved hy decomposition into
a series of superimposed laminae, or sheathed cones, and this
fact has been regarded as indubitable proof of their original
formation hy successively transuded layers of dental substance.
Nevertheless, microscopic sections of such teeth have uniformly displayed
a structure of tubes running in a direction diametrically opposite
to the plane of the lamellae. These lamellae are, in fact, due, not to
successive transudations from the free surface of the formative pulp,
hut to alternations of states of comparative activity and repose in the
organizing processes by which the stratum of the pulp, in immediate
connection with the last formed or basal surface of the tooth, is prepared
to receive the hardening salts, combined with alterations in the
nature itself of the organizing process. These alternate changes are
indicated by the layers of abundant calcigerous cells which are scattered
through the intertuhular tissue of the tooth in planes corresponding
with the apparent stratification of the dental substance, and with which
cells angular inflections of the calcigerous tubes frequently correspond.
A separation of corresponding layers of parallel aggregated segments
of calcigerous tubes, in the tusks which manifest the preceding structure,
is the usual result of their decomposition.
In the compound dental plate of the Diodon, similarly parallel and
aggregated series of short calcigerous tubes are separated by thin layers
of a cellular hone ; and by decomposition, such a tooth would, in like
manner, exhibit the lamellated structure; but the lamellae in this case, as
in the tusk of the elephant, or the conical molar of the cachalot, equally
present an organized structure of aggregated calcigerous tubes, directed
more or less at right angles to the plane of the lamella, and
indicating that higher mode of development by calcification of the
pulp, which it is the chief object of the present researches to exemplify.
The exposed margins of the upper and lower jaws of the Diodon,
which appear to be covered with a thick irregular layer of the same
dense white dental substance as that of the posterior triturating
masses above described, owe their apparently simple character to a
still more complicated structure. They consist of a series of narrow
flattened denticles lying horizontally, and at right angles to the anterior
surface of the jaw ; so that those of the exterior row of one
jaw have their sides opposed to the corresponding row in the
opposite jaw when the mouth is closed. These denticles are developed
in a cavity, between the outer and inner walls of the jaws,
the floor of which is formed by a thin cribriform osseous plate, separating
the cavity containing the teeth (PI. 38, fig. 2, 6) from the wide
vascular canal fcj which occupies the substance of the jaw. At the
bottom of the dental cavity, the denticles are seen in different stages
of formation, unattached, but closely packed, and with their lateral
margins overlapping each other. They are of an oval form, with
the under surface, or that next the floor of the cavity, slightly concave
and smooth : the opposite surface convex, and honeycombed:
the denticles gradually diminish in size from the middle to the
outer ends of the dentated margin. As their formation reaches its
completion, the denticles become anchylosed to each other and to
the osseous parietes of their cavity by ossification of the capsules of
the calcified pulps. The thin layer of bony matter or cement enclosing
the denticles is soon worn away when they reach the margins
of the jaws, the irregularity of which is caused by the alternately
overlapping arrangement of the exposed denticles.
The order of development and succession is the same in the marginal
as in the posterior teeth, they ascend in the lower and descend
in the upper jaw, hut are pushed on in the horizontal instead
of the vertical direction. The chief difference is, that whereas in the
posterior dental tubercle there are only two broad lamelliform teeth
in the same plane, in the marginal series there are upwards of forty
narrower denticles.
Cuvier observes that the Tetrodons differ from the Diodons inasmuch
as they have no posterior triturating disk, but only the marginal
plates ; and have the jaws divided, each into two portions by a den-
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