branchiate Batrachians which stand at the lowest step of the Reptilian
Class, and not only the superior maxillary teeth, but the bones
themselves are absent in the Siren, Menobranchus and Proteus.
76. Siren.—In the Siren (Siren lacertina, Linn.) the lower margin
of the intermaxillary bones, and the sloping anterior and upper margin
of the lower jaw are trenchant, and are each encased in a sheath of firm
albuminous, minutely fibrous tissue, harder than horn, (PI. 62,
fig. 1). The bones thus armed slide upon each other like the blades of a
pair of curved scissors, when the mouth is closed, and are well adapted
for dividing the bodies of small fish, aquatic larvae, worms, &c.
The horny substitute for teeth on the lower jaw is supported by
the bony element corresponding with the premandibular of the
lepidosiren and other fishes. A second osseous piece, applied to the
inner surface of the ramus of the jaw, and representing the .splenial
or opercular element in the jaw of the crocodile, is beset with
numerous minute pointed teeth, arranged in short oblique rows, and
directed obliquely backwards. The palatal surface of the mouth presents
on each side two flat, thin and moderately broad bones, forming
an apparently single oblique oval plate, which converges to meet its
fellow at the anterior part of the palate, so as conjointly to constitute
a broad rasp-like surface in the form of a chevron, (PI. 62, fig. 2).
The anterior long plate on each side, which may be regarded
as the representative of the divided vomer, supports six or seven
oblique rows of small pointed retroverted teeth ; the smaller posterior
plate, probably the homologue of the pterygoid, is beset
with four rows of similar teeth : there being thus ten or eleven
rows on each side of the palatal chevron. The number of denticles
in the middle rows is eleven or twelve ; they become fewer
in the anterior and posterior rows : they are all of similar size and
form, corresponding with those of the lower jaw to which they are
opposed.
The condition of the dental system in this, the lowest of the class
of reptiles, is not without interest independently of the absence of
the superior maxillary teeth, and of the presence of the palatal and
inferior maxillary ‘ dents en cardes.’ If, for example, the dense sheath
of the trenchant anterior parts of the upper and lower jaws had
been completely calcified and converted into hard dentine, the correspondence
between the siren and the lepidosiren would have been
very striking in this part of their structure; but the maxillary
sheaths of the siren being composed of horn, and being, moreover,
easily detached from the subjacent bones, much more closely resemble
the deciduous mandibles of the tadpoles of the higher Batrachians.
(1)
77. Axolotl.—The ichthyic character of rasp-like teeth, aggregated
in numerous series, is manifested also in the Axolotl upon the palatal
region of the mouth, and upon the splenial or opercular element of
the lower jaw ; but the superior maxillary bones are here developed and
support teeth (PI. 62, fig. 4). The premandibular and the intermaxillary
bones (a), instead of presenting the larval condition of the
horny sheath, have their alveolar border armed with a single row of
small, equal, fine, and sharp pointed denticles, which are continued
above, along the maxillaries (6); thus establishing the commencement
of the ordinary batrachian condition of the marginal teeth of the buccal
cavity. The dentigerous bones of the palate consist of two plates on
each side, as in the siren; the anterior pair, or vomers, (c), converge
and meet at their anterior extremities ; the minute denticles which they
support are arranged quincuncially: the posterior pair of bones, (d), are
continued backwards, according to the usual disposition of the pterygoids,
to abut against the tympanic or quadrate bones ; the denticles are
confined to the anterior part of their oral surface and resemble in their
arrangement and anchylosed attachment those of the palatal series of
which they form the posterior termination.
78. Menobranchus.—Although in this genus the superior maxillaries
and their teeth are wanting, an advance to a higher type of dentition
(1) “ Leur bord est tranchant et garni dans l’animal frais d’une gaine presque cornée, qui
se détache aisément de la gencive, et qui a son analogue dans les têtards de grenouille.”—
Cuvier, Ossem. FosSi.JEd. 1837, x, p. 341. The combination of trenchant with lacerating
rasp-like dental instruments in the jaws of the Siren was recognized by Ellis, who has given
a figure of them in the 56th volume of the Philosophical Transactions, PI. IX. c. He says,
“ The mouth is small in proportion to the body; but its palate and inside of the lower jaw
are well provided with many rows of pointed teeth ; with this provision of nature, added to
the sharp exterior bony edges of both the upper and under jaw, the animal seems capable of
biting and grinding the hardest kind of food.”