which the parietes of the medullary canal have been divided parallel
to its course, the calcigerous tubes are cut through transversely, and
their areae, from the calcareous nature of their contents, are seen like
fine dots.
SPATULARIzE.
2 Polyodon.—Of the other free-gilled Plagiostomous fishes theSte-
rionidce or family of Sturgeons and the Planirostra are edentulous. The
Polyodon, which belongs to the same natural family as the Planirostra,
viz : the Spatulariee of Müller, has small recurved teeth on the maxillary
and anterior branchial cartilages, at least, at the earlier periods of
life. The specimens of Polyodon, (e. g. in the Parisian Museum), which
exceed three feet in length are, like the Planirostra, edentulous ; but
in a Polyodon of the same species about one foot long, there are two
rows of small and slightly recurved teeth in the upper, and a single
row of similar teeth in the lower jaw: similar teeth occur likewise on
the two anterior branchial arches, where these join the tongue; and
again on their expanded extremities which are attached to the palatal
region of the mouth :(1) here, therefore, there are both maxillary and
branchial teeth. Thus the Spatulariee not only exhibit a tendency to
the structure of the ordinary osseous fishes in the condition of their
branchial apparatus, hut likewise in that of their dental system ; the
Polyodon being the only plagiostomous fish which has both maxillary
and branchial teeth.
C H A P T E R IV.
T E E T H OF THE GANOI D F I SHES .
LEPIDOIDS.
27. The consideration of the teeth of this family of Ganoid
fishes is not without interest, although as compared with the Pla-
giostomes, few modifications require to be noticed. All the representatives
of the family have disappeared from the present theatre of
vital phenomena ; yet although the period when they existed is the
(1) Muller, Vergleichende Anatomie der Myxinoiden, p. 150.
I earliest of those in which palaeontologists have recognized the traces
I of animal life, the same peculiarities are manifested in the structure
I and disposition of their dental organs, as in those of many of
I the existing osseous fishes. The teeth are always present on the
I palatine hones, and are arranged in several rows on the alveolar
I margins of the intermaxillary and premandibular bones ; those of the
I outer row are the largest, and are sometimes in the form of obtuse
I cones ; the posterior ones are small, simple and close-set, like the
I bristles of a brush.
In the genus Amblypterus all the teeth are of the latter kind, and
1 their minute slender character is the more remarkable on account of
I the disproportionate magnitude of jaws. In the genera Palmoniscus
I and Semionotus, the maxillary teeth also resemble a fine brush ; indi-
I eating, Dr. Buckland observes, “ the habit of these fishes to have been
I to feed on decayed sea-weed, and soft animal substances at the bottom
I of the water.”
In the large enamel-scaled fishes of the Lias formation belonging
I to the genera Dapedius and Tetragonolepis, the teeth are stronger and
I better developed, especially the exterior ones. In the Tetragonolepis,
I the summit of the crown is simply pointed, but in the Dapedius it is
notched or bifurcate ; and this modification is not due to usage or
I compression, for the teeth of all the rows and in both jaws exhibit the
I same character, which is also well marked in the successional teeth
I that have not come into place. In Dapedius Orbis, the teeth of both
| jaws are strongly dilated and compressed from before backwards,
I at their summits, and resemble a chisel with a notched cutting
I edge.(l)
In the extinct genus Lepidotus, the mouth was small and
1 the jaws short and rounded. The intermaxillary bones form only
I the anterior part of the upper margin of the mouth, the maxillaries
I completing the posterior part of that border. The margins of both
I these bones, according to M. Agassiz, are beset with small teeth ; the
I outer row presenting the form of circular obtuse cones ; and within
I these are many rows of smaller sessile hemispherical teeth, more or
(1) M. Agassiz assigns pterygoid, as well as palatine and intermaxillary teeth to the genus
I Dapedius, t ii, p. 187.