fishes, especially the Scarus and Diodon, which have been cited as examples
of the fourth modification of the dental structure. These teeth
strike fire with steel, yet they present an organized structure of minute
complexity, and the calcigerous tubes are nowhere so numerous,
so minute, so beautifully ramified and interlaced together.
It has already been observed that the prismatic maxillary denticles
of the Scarus, being compacted together side by side and with their
single medullary canals parallel to each other, produce a compound
dental plate analogous to those wdiich were first cited in illustration of
the present subject. In the Diodon, each denticle, which is composed
as in the Scarus of a single system of minute calcigerous tubes, assumes
the form of a thin plate : the pulp cavity instead of being contracted
into a tube, as in the elongated teeth, is here spread over the under
surface of the dental lamella ; the calcigerous tubes proceed in a direction
more or less vertical to the upper surface (in the teeth of the
lower jaw); and the compound dental mass results from the superposition
and successive development of similar plates, separated
only by a layer of thin bone or ccementum. In the pharyngeal teeth
of the Scarus, the denticles are also more or less lamelliform, but their
position is vertical, and they are joined side to side by means of the
intervening cement or the ossified capsule : compound dental masses
similarly constructed are present in the capybara, elephant, and
others of the mammalia, generally cited as affording examples of the
most complex teeth.
9. Development.—The teeth of fishes are formed according to the
general laws of dental development already discussed ;(1) but the process,
in many instances, does not extend beyond the earlier and simpler
stages observable in the higher classes of animals. In all fishes, as
in other vertebrate animals, the first step is the production of a simple
papilla from the free surface of either the soft external integument,
as in the young Pristis,(2) or of the mucous membrane of the mouth, as
Cl) Introduction, p. vi.
(2) A very close analogy exists between the dermal bony tubercles and spines of the cartilaginous
fishes and their teeth. The thick enamelled scales of the ganoid fishes of Agassiz exhibit
an organization similar to that of the teeth: the system of minute parallel tubes, with their
branches and anastomoses, in the thick scales of the extinct lepidotus, is as complicated as m
in the rest of the class. In these primitive papillae there can be very
early distinguished a cavity containing fluid, and a dense membrane,
(membrana propria pulpij surrounding the cavity, and itself covered by
the thin external buccal mucous membrane, which gradually becomes
more and more attenuated as the papilla increases in size. In some
fishes, as the sharks and rays, the dental papillae do not sink into the
substance of the vascular membrane from which they grow, but become
buried in depressions of an opposite fold of the same membrane
; these depressions enlarging with the growth of the papillae,
and forming the cavities or capsules in which the development of the
tooth is completed. They differ from the capsules of the matrix of
the mammiferous tooth in having no organic connexion with the
pulp, and no attachment to its base: the teeth when fully formed are
gradually withdrawn from the above described extraneous capsules, to
take their place and assume the erect position on the alveolar border
of the jaws.
Here, therefore, is represented on a large and, as it were, persistent
scale, the first and transitory papillary stage of the development
of the mammalian teeth; and the simple crescentic cartilaginous
maxillary plate, with the mucous groove behind it containing
the germinal papillse of the teeth, offers in the shark a magnified representation
of the earliest condition of the jaws and teeth in the
human, embryo.
In many fishes, as the lophius and pike, the dental papillse become
buried in the membrane from which they arise, and the surface to
which their basis is attached becomes the bottom of a closed sac.
But this sac is never lodged in the substance of the jaw, the development
of the tooth being completed in the tissue of the thick and soft
gum or mucous membrane from which the papillse were originally
developed: hence teeth in various stages of growTth are frequently
brought away with that membrane when it is reflected from the jawbone.
The ultimate fixation of the teeth, so formed, is effected by the
development of ligamentous fibres in the submucous tissue between
the jaw and the base of the tooth ; which fibres become the medium
many teeth, and equally militates against the theory of formation by transudation of layers
being applied, at least, to the ganoid scales.