cavity containing the remains of the pulp, and the tooth is
loose, or is suspended only by its pulp and capsule I still lower
down, the capsule is observed in different stages of ossification;
and the tooth becomes thereby anchylosed, first by its anterior, and
afterwards by its posterior extremity, to the opposite walls of the
formative cavity. The pulp-cavity becomes diminished in size by
the progressive calcification of its organized contents, and at length
the tooth, by the complete ossification of its external capsule,
becomes adherent laterally to the contiguous teeth, as well as to
the walls of the dental cavity by its extremities: thus the dense,
exposed anterior portions of the premaxillary and premandibular
bones are converted into an aggregated and anchylosed mass of
teeth.
When the microscopic structure of one of the maxillary denticles
is examined in a longitudinal section, the conical pulp-cavity may
be traced to near the coronal end of the tooth. The calcigerous tubes,
which at their origin do not exceed ji-0 of an inch in diameter, are very
closely aggregated, being separated by intervals equal to only one
and a half of their own diameters ; those at the base of the pulp-cavity
descend, and on one side of the denticle, incline inwards, tending to
close that cavity ; the greater number of the calcigerous tubes proceed
directly outwards, at right angles to the sides of the denticle.
near the coronal surface they begin to bend towards that end,, and
to pass more obliquely from the pulp-cavity, until they assume,
in the centre of the crown, a direction parallel to the axis of the
tooth. Besides the primary curvatures, which are most marked at the
two extremes of the denticle, all the calcigerous tubes present the
most regular and graceful undulations throughout their entire course:
these undulations require a compound lens of |th inch focus to he
distinctly seen, and, with the fine lateral branches of the tubes, form
no mean test of the powers of the microscope employed. The pulp-
cavity becomes finally occupied by a coarse cellular bone, and a thm
coating of the same kind of tissue invests the exterior of the denticle.
The general disposition of the calcigerous tubes, and the form of the
detached denticle and its pulp-cavity are figured in PI. 50.
The thick external enamel-like layer of each denticle, which
is indicated by its lighter colour in the plate, consists likewise of
minute fibres or tubes, which are thickest and straightest at the
periphery of the tooth, but bend and cross each other as they approach
the central tubes, appearing to interlace and anastomose at
the boundary line.
The outer wall of the common alveolar cavity in which the
denticles are developed, is much weaker than the dense and compact
inner wall; it becomes thinner as it approaches the margin
of the jaws, and disappears at different distances in different
species of Scan, before it reaches that margin. Where it exists
at the base of the jaws, it is sometimes, as in Scarus muricatus,
(PI. 49, fig. 1), perforated by numerous small foramina, through
which foramina, in the recent fish, processes of the external periosteum
are continued to the analogous membrane, lining the dentigerous
cavity, and forming the capsule of each denticle. These
processes are analogous to the gubernacula of the second series of
teeth in the mammalia, and, like them, serve to conduct the new teeth
to the exterior of the jaw. The growing denticles become elongated
by the addition of successively calcified portions of their pulp to
their basal or posterior extremities ; the opposite end exerts a proportional
pressure against the circumference of the foramen, and,
causing its absorption, begins to protrude. The tuberculate crown of the
denticle is exposed about the time when its sides have become anchylosed
to those of the previously protruded row. Thus from the
close apposition of the protruding denticles, the whole of the outer
parietes of their common alveolar cavity, subjected to the stimulus of
their pressure, is finally removed, and is replaced by the pavement of
mutually anchylosed teeth.
In certain species of parrot-fish one or more of the denticles, at
the posterior part of the intermaxillary bones, are longer than the
rest, and their coronal extremity is produced into a sharp and
sonietimes recurved point; these teeth extend beyond the pavement
formed by the other denticles and constitute a weapon of offence, as
in the Scarus aurofrenatus, Sc. vetula, Sc. quadrispinosus and the
species figured in PI. 49, fig 2.
In some other species, as the Scarus jlavescens cf Bloch and
i 2