being thus independent of the superincumbent weight, freely yields
to the absorbent process consequent on the growth of the new tooth;
and before the latter becomes subjected to any pressure from above,
its formation has been sufficiently perfected to enable it to sustain that
pressure without injury.
The lateral walls of the cavities containing the reserve teeth, to
which the pressure is transferred from the margins of the sockets of
those in use, consist of a much denser osseous tissue than the other
parts of the pharyngeal bone.
The Wrasses feed on testaceous Mollusks, Crustaceans, Echini,
&c., which they seize with their long anterior conical teeth, and
crack and bruise by means of their powerfully armed pharyngeals.
Some of the sub-genera of Labroids, Epibulus and Clepticus, have the
intermaxillary bones provided with very long ascending processes,
and an arrangement of muscles, which producing a rotatory motion
of the maxillaries and a sudden descent of the intermaxillaries,
accompanied at the same time with a protraction of the lower jaw,
produces a tubular elongation of the whole mouth, and adds to their
power of catching the smaller marine animals.
48. Scarus. (1)—There is a genus of fishes adapted to browse on the
lithophytes which clothe, with a richly tinted verdure, the bottom of
the sea, as the ruminant quadrupeds crop the herbage of the dry land.
The irritable bodies of the gelatinous polypes which constitute
the food of these fishes retract, however, when touched, into the starshaped
cavities of their stony support, and the Scari consequently
require a dental apparatus strong enough to break away and reduce
to a pulp these calcareous recesses. Their jaws are, therefore, prominent,
short, and stout, and the exposed portions of the intermaxillaries
and premandibulars are shaped like the beak of a parrot,
whence the name of “ parrot-fish,” usually given to these brightly-
tinted species of the Labroid family. But the mandibles instead of
being sheathed with horn, are encased by an extremely dense and
singularly complicated dental covering.
Each intermaxillary bone presents a triangular form, with the
superior and posterior angles produced, and separated by an oblique
(1) PI. 49, 50, 51.
excavation or concavity, in the middle of which there is a small oval
articular depression, to which a corresponding convex trochlea of
the maxillary bone is adapted. Powerful muscles are inserted into
the two produced angles of the intermaxillary bone, by means of which
it is worked upon the hinge-like joint above described. The
premandibular piece of the lower jaw differs from the intermaxillary
bone above in the absence of the posterior median process, but
in other respects it closely resembles that bone in form.
When the intermaxillary and premandibular bones are viewed
from their outer side, they appear to be edentulous, and to have
their osseous texture converted, at the anterior and outer part,
into an enamel-like substance, with a surface chequered by small
lozenge-shaped smooth or tuberculate plates arranged in a quin-
cuncial order, (PI. 49, fig. 1), When viewed from the inside, (PI. 49
■ fig- 3), the rregular trenchant margin of these bones is seen to be
composed of short and thick sub-conical four-sided columns, placed
almost vertically to the outer surface of the jaw, along a great part
of which surface similar columns are arranged, which form by their
exposed bases the tesselated surface above mentioned. These small
columns are the teeth ; the exposed base is the crown, and the opposite
contracted end, which is slightly excavated, corresponds with the
,anS(l)- If a vertical section of the jaw be made, as in fig. 3, these teeth
| or denticles will be exposed in various stages of formation in a cavity
(i) below the dental series in the premandibulars, and in the reverse
situation in the intermaxillaries. The formative quadrilateral pulps
are contained in this cavity, with their bases attached to the vascular
membrane lining its posterior wall, and with their free extremities projecting
horizontally forwards towards the anterior or outer wall.
Alveolar or capsular processes of vascular membrane are continued
backwards from the outer wall and inclose the pulps, like the
|capsule of the matrix of ordinary teeth. At the uppermost part of
the dentigerous cavity in the upper jaw, the broad anterior extremity
[of the columnar tooth is seen to be formed by calcification of the
summit of the pulp : a little lower down, the whole length of the
tooth is completed, but the base is excavated by a conical
(1) Cuvier, Leçons d’Anatomie Comparée, Nouv. Ed. Paris, 1836, tom. iv, p. 226.