like coating of the tooth receives numerous similar parallel ramuscules
from the stratum of calcigerous cells which forms the boundary
between this external layer and the ivory of the tooth. The pulp-
cavity, which has a compressed ovate section, is gradually contracted
and finally obliterated in the basis of the tooth, where it divides into
numerous minute canals which subdivide and anastomose as they
penetrate deeper into the jaw, and finally terminate in minute tortuous
canals, which become continuous with those of the coarse
osseous substance in which they are imbedded. All the preceding
branches of the pulp-cavity are the centres of radiation of as many
systems of calcigerous tubes, similar in size to those of the body
of the tooth. The primary curvatures of these tubes are shorter, and
the interspaces somewhat wider; the calcigerous cells in which the
fine branches of these calcigerous tubes terminate, are also coarser.
The interspaces of the divisions of the base of the tooth are occupied
with the coarse cellular bone of which the jaw is composed.
The teeth of the Rhizodus thus include two very different types
of structure, which pass insensibly into one another; the exserted
body of the tooth is like the dense ivory of the ordinary mammiferous
or saurian tooth, the inserted basis presents the compound structure
of the teeth of the Myliobates and Orycteropus. Each division of the
tooth’s basis, however, presents in the Rhizodus greater strength and
density than the corresponding parts of the tooth of the Myliobates,
in consequence of the greater minuteness, increased number, and
more aggregated and parallel disposition of the calcigerous tubes, and
the smaller diameter of the medullary cavity in the former species.
The advantage which was obtained for the teeth of the extinct
carnivorous Sauroid by the root-like implantation above described,
in resisting displacement and fracture, is too obvious to need further
illustration. Finally, as the teeth of no species of reptile or fish are
similarly subdivided and implanted in the jaws, the teeth of the
Rhizodus, notwithstanding the organization of the crown, may be
readily and certainly distinguished from them.
The large conical laniary teeth of the extinct genera Holoptychus
and Megalichthys present a similar structure to that of the tooth
above described. The whole body of the tooth is composed in
Megalichthys of minute close-set calcigerous tubes, having a diameter
of -rrVirth of a line, with interspaces of twice that diameter. The
calcigerous tubes have a minutely undulated course, and pass
in nearly a straight line from the internal to the external surface of
the tooth ; the pulp-cavity extends about half-way through the body
of the tooth, and has a narrow elliptic transverse section ; it becomes
gradually smaller at the base of the tooth, and there branches out
into several ramifications, which are continued into the cylindrical processes
of the dental substance, and these are imbedded, like so many
piles, in the coarse osseous texture of the jaw.
GYMNODONTS.(l)
30. To the theory of dental development by intus-susception or the
deposition of calcareous tubes in the pulp s substance, as opposed
to that of apposition or the transudation of layers of calcareous
matter from the pulp’s surface, may be objected the structure
and mode of formation of the compound teeth of the gymnodont
fishes, as described by Cuvier(2) and Von Born. (3) In the
Diodon, more especially, the lamellated structure of the tooth, and
its reproduction by successive transudation of layers from a persistent
pulp, were supposed to be clearly demonstrated in the
broad rounded triturating tubercle which is situated behind the
alveolar border of the jaws. The exposed surface of this tooth presents,
in fact, a series of transverse and parallel striæ (PI. 38, fig. 1),
which, in a vertical section lib. fig. 2) are seen to be the margins of
thin, superimposed, horizontal, and slightly flexuous plates, which have
been partially abraded by trituration in an oblique plane. The superior
layers are the most worn, and are evidently, as Cuvier
observed, the oldest ; in proportion as they descend, in the lower jaw,
they increase in breadth ; and finally, instead of being soldered
together, they become detached, thinner, and of a more friable texture,
the lowest and incompletely developed plates lying loosely in the
cavity of the jaw beneath the superincumbent dental mass lib. fig. 2 , a).
If a vertical section of the tubercle be made on one side of the median
(1) yvpvos naked, oSovç a tooth, fishes with teeth exposed.
(2) Leçons d’Anatomie Comparée, 1st and 2nd editions.
(3) Heusinger’s Zeitschrift, 1827.