the dentigerous plate on each side ; the intermediate space is occupied
exclusively by the larger lamelliform or wedge-shaped teeth,
set vertically in the bone, and arranged transversely in alternate and
pretty close-set series.
The pharyngeal denticles are developed in wide and deep cavities
in the substance of the posterior part of the lower, and of the anterior
part of the upper pharyngeal bones. Each denticle is inclosed in its
proper capsule which contains an enamel-forming pulp and a dentinal
pulp, in close cohesion with each other, and with the thin external
capsule. The teeth exhibit progressive stages of formation as they
approach the posterior part of the upper and the anterior part of
the lower pharyngeal bones e as their formation advances to completion
they become soldered together by ossification of their respective
capsules, and soon afterwards are anchylosed by ossification of the
base of the dentinal pulp to the pharyngeal bone itself. The line of
demarcation between the dentified and ossified portions of the pulp
is well defined, so that when the pharyngeal bone and teeth are
sawn through vertically the fully formed teeth appear as hollow cones,
set upon wedges of bone, as shown in PI. 51, fig. 1.
The dentine of the pharyngeal teeth of the Scarus consists ol
calcigerous tubes and a clear intermediate substance. The calci-
gerous tubes average a diameter of 5000 th °f an inch, and are
separated by interspaces equal to twice their own diameter. The
course of these tubes is shown in PI. 52, fig. 2, b, in which they
are exposed by a vertical section through the middle of one of the
inferior denticles. They all, on leaving the pulp-cavity, form a
curve with the convexity turned towards the base of the tooth, and
then bend slightly in the opposite direction ; the sigmoid curve being
most marked in the calcigerous tubes at the base of the denticles,
whilst those towards the apex become longer and straighter. Besides
the primary curvatures exemplified in the figure, each calcigerous
tube is minutely undulated ; it dichotomizes three or four times near
its termination, sends off many fine lateral branches into the clear
uniting substance (PI. 52, fig. 3), and finally terminates in a series
of minute pells and inosculating loops at the line of junction with
jthe enamel.
This substance (PI. 52, c.) is as thick as the dentine and consists
of a similar combination of minute tubes and a clear connecting
substance. The tubes may be described as commencing from the
peripheral surface of the tooth, to which they stand at right angles,
and, having proceeded parallel to each other half way towards the
dentine, they then begin to divide and subdivide, the branches
crossing each other obliquely, and finally terminating in the cellular
boundary between the enamel and dentine.
The teeth, which present this complex structure, are successively
developed at one extremity of the bone, in proportion as they are
worn away at the other; not, however, as Cuvier describes, from behind
forwards, in both upper and lower pharyngeal bones, (1) but in opposite
directions in the opposite bones, the course of succession being from
before backwards in the upper, and from behind forwards in the lower
pharyngeal bones. In the progress of the attrition to which they
are subjected, the thin coat of cement resulting from the ossification
of the capsule is first removed from the apex of the tooth,
then the enamel (c) constituting that apex, next the dentine (b), and
finally the coarse cellular central bone (a), supporting the hollow-
wedge-shaped tooth, and thus is produced a triturating surface of
four different substances of different degrees of density. The enamel,
being the densest element, appears in the form of elliptical transverse
ridges, inclosing the dentine and central hone, and external to the
enamel is the cement which binds together the different denticles.
The Comparative Anatomist, conversant with the modifications
of the dental system in the mammiferous class, cannot but be struck
with the close analogy between the adherent pharyngeal denticles
of the Scarus and the complicated grinders of the elephant, both in
form, structure, and in the reproduction of the component denticles
in horizontal .succession. But in the fish, the complexity of the
triturating surface is greater than in the mammal, since, from the
mode in which the wedge-shaped denticles of the Scarus are implanted
upon, and anchylosed to, the processes of the supporting bone,
this likewise enters into the formation of the masticatory surface when
the tooth is worn down to a certain point.
(1) Histoire Naturelle des Poissons, tom. xiv, p. 1 16.