inwardly projecting convex ridge. The masonry of this internal
buttress and of the dome'itself, is composed of hollow columns, every
one of which is placed so as best to resist or transmit in the due
direction the superincumbent pressure. The advantages gained by
this beautiful example of animal mechanics will be explained when the
dental system of the labroid fishes is described.
In another case, in which long and powerful piercing and lacerating
teeth were evidently destined, from the strength of the jaws,
to master the death-struggles of a resisting prey, we find the broad
base of the tooth divided into a number of long and slender cylindrical
processes, which are implanted, like piles, in the coarse osseous substance
of the jaw ; they diverge as they descend, and their extremities
bend and subdivide like the roots of a tree, and are ultimately lost
in the bony tissue. This mode of implantation of the teeth, which
I have detected in a large extinct sauroid fish (Rhizodus) ,(1) is, perhaps,
the most complicated which has yet been observed in the animal
kingdom.
6. Substance.—The teeth of fishes, in respect to their substance,
present various degrees of density and complexity. In most
of the chsetodonts they are flexible and elastic, of a yellowish, shining,
and subtransparent tissue. The labial teeth of the helostome are
also of this kind, as are also the anterior maxillary teeth of the gonyo-
donts, and of the percoid species, hence called Trichodon. In the cyclos-
tomes, the teeth consist of an albuminous tissue, of a somewhat denser
nature. The upper pharyngeal molar of the carp, consists of a peculiar
brown and semi-transparent tissue, harder than the true horny teeth
of the lamprey.
The greater number of fishes have their teeth composed of an
osseous substance, somewhat denser than the jaws to which they
are affixed. In some instances, as in the teeth of the flying-fish
(Exoccetus), and sucking-fish {Remora), the substance of the tooth is
uniform, and not covered by a layer of a denser texture. In others, as
the shark, sphyrmna, &c., the tooth is coated with a dense, shining,
enamel-like substance ; but this is not true enamel, nor the product
of a distinct organ ; it differs from the body of the tooth only in the
greater proportion of the earthy particles, their more minute diffusion
0 ) Pi. 36.
through the gelatinous basis, and the more parallel arrangement of
the calcigerous tubes ; but it is developed in and by the same matrix,
and, resulting from the calcification of its external layer, is the first
part of the tooth which is formed. In the Sargus and Batistes, the
dentine, or proper osseous substance of the tooth, is harder than that
of the fishes last cited, and is covered with a thick layer of a denser substance,
developed by a distinct organ, and differing from the enamel of
the higher animals only in the more complicated and organized mode of
deposition of the earthy particles. The ossification of the capsule of the
matrix gives the enamel of the teeth of the file-fish, and some others,
a thin coating of a third substance analogous to the “ cæmentum,
or crusta petrosa,” of the mammalian teeth. And in the pharyngeal
teeth of the parrot-fish, a fourth substance is added to the structure
of the tooth by the coarser ossification of the pulp, after its peripheral
portion has been converted into the dense ivory. The teeth, thus
consisting of dentine, enamel, cement, and coarse bone, are the most
complicated as regards their substance that have yet been discovered.
7. Chemical composition.—With respect to the chemical composition
of the teeth of fishes, little remains to be added to what has been
stated on this subject in the preliminary general observations. The
animal base of the horny teeth of the cyclostomes is albuminous, as
in true horn ; that of the calcified teeth is gelatinous, and the proportion
of gelatin to the usual hardening salts, diminishes as their
density increases. According to the analysis of Lassaigne,(l) the teeth
of the shark yield
Phosphate of lime . . 52, 6
Carbonate of lime . . 13, 9
Animal matter . . 33, 5
100, 0
The inferior pharyngeal teeth of the carp contain
Phosphate of lime . . 49
Carbonate of lime . . 16
Animal matter . . . 35
100,0
(1) Berzelius, Traité de Chimie, par Esslinger 1828, tom. vii, p. 480.