tated suture. In the upper jaw of many species of Tetrodon, there is
a rudimental posterior dental series, consisting of three or four plates
which project downwards and backwards from the base of the intermaxillary
bones, and intercept a space in which the apex of the
lower jaw is received when the mouth is closed. In Plate 39, fig. 1,
is given a figure of the beak-like jaws of the Tetrodon lineatus, showing
the median suture, the lines of stratification of the marginal dental
plates, and, at (a), the posterior lamelliform teeth above described. The
marginal lamelliform teeth are from ten to twelve in number in
each half of the mandible ; the innermost are the broadest, and they
become narrower as they pass outwards. The intervening portions,
or bases of their respective pulps remain longer in the unossified state
than in the Diodon. The microscopic structure of the teeth of the
Tetrodon closely agrees with that of the dental tubercle of the Diodon.
SCLERODERMS.
31. The teeth in the file-fishes (BalistesJ are limited to the intermaxillary,
premandihular and pharyngeal bones.
In the Batistes forcipatus, PI. 40, the teeth of the upper jaw are
fourteen in number, and are arranged in two rows, seven in each intermaxillary
hone, four in the front row and three behind. In the
lower jaw there are eight teeth corresponding with the front row above.
The anterior or external teeth of the upper resemble those of the
lower jaw ; they are strong, conical, subtrihedral, hollow at the base,
which is obliquely truncated, and rounded and obtuse at the apex.
The mesial pair is slightly curved, and is the largest; the rest decrease
in size to the outermost. The external facet of each tooth is
covered with a smooth, dense, enamel-like substance, which, towards
the apices of the teeth, presents a yellow colour, and calls to mind the
peculiar colour of the enamel in some of the Rodentia. These outer
maxillary teeth are arranged in close contact with one another
(PI. 40, figs. 1 and 7). The form of the alveolus in which the base is
fixed, is peculiar in the dental system, resembling rather the surface
of attachment for the claw in the ungueal phalanges of the feline
quadrupeds. A conical process of the bone rises from the middle of
the alveolar depression, and is adapted to the cavity in the base of the
tooth (PL 40, figs. 3 and 5). The circumference of the base of the
fully formed tooth is attached by a slight anchylosis to the margin
of the alveolus, but the confluence of the tooth with the hone is much
less complete than in many other fishes.
There would seem to he a constant and pretty quick succession of
these teeth, for in all the jaws of different species which I have examined,
there were the crowns of a second or new series of teeth,
generally pretty far advanced in their development. The successors
of the external teeth of the right intermaxillary bone, exposed by removing
the outer wall of their alveoli, are represented in situ in fig. 5
of Plate 40 : Jos) points to the osseous tubercle on which the tooth
about to be displaced was fixed; the absorbent process has commenced
at its apex I at (b) the corresponding bony tubercle has been
thus entirely removed, and the obtuse apex of the new tooth has
protruded in the socket of that which it has displaced. The cavities
containing these teeth communicate with the exterior of the jaw
by foramina, situated as in most other fishes, on the outer side of the
base of the teeth in place (PI. 40, figs. 1 and 3).
The teeth of the posterior row, which are peculiar to the upper
jaw, are six in number, three in each intermaxillary bone:
they present the form of elliptical plates, compressed laterally,
rounded at the base, and slightly pointed at the apex. The
anterior tooth is the largest, measuring in Batistes forcipatus six
lines in length, and three in breadth, hut scarcely half a line in
thickness; the two other teeth progressively diminish in size (PI. 40,
fig. 4). These posterior teeth lie in close juxtaposition with
the outer row, and like the posterior small upper incisors of the hare
and rabbit, receive part of the appulse of the inferior teeth. They
are affixed by a very oblique and slightly excavated base to a shallow
alveolus, having a convex rising of bone in its middle (PI. 40, fig. 6).
They are also deciduous, and the presence of well developed reserve-
teeth in cavities of the jaw, immediately internal to those of the exterior
row, would indicate that the succession of the teeth of the inner
row is likewise unlimited. The foramina leading to the cavities of the
successional teeth are seen immediately above the bases of the teeth
in place. The germs of the successors of the inner row of teeth exg
2