Petromyzon marims, the innermost teeth of four of the lateral series on
each side are bicuspid, or consist each of two cones, which are
confluent at the base. There are twenty converging rows of labial
teeth in this species, and from four to eight teeth in each row.
The single tooth supported by the palatal cartilage(l) and analogous
to that in the myxinoids, consists here of two horny cones,
placed in the transverse direction, and joined, in the Petr, marinus,
in the median line. In the lampern (Petr, jluviatilis) the cones are
more remote. The matrix of this tooth is hollow at the base, and
is supported on a conical process of the palatal cartilage, which
Cuvier describes as the upper jaw. The broad bicuspid palatal tooth
is opposed by the dentated semilunar horny plate, with which the
cartilage, representing the lower jaw,(2) is sheathed. This plate
consists of eight conical teeth, laterally united together; its reproductive
matrix is fixed upon a prominent semilunar ridge at the
anterior part of the mandibular cartilage.
The lingual teeth consist of three dentated horny plates, the
dentations being much smaller than in the palatal or mandibular
plates, or than in the lingual plates of the myxinoids. But their
analogy with the latter can be readily traced, the third or posterior
lingual plate of thé lampreys evidently corresponding with the two
posterior lingual plates of the myxinoids conjoined.
Each of the anterior lingual plates is slightly concave, with the
mesial extremity abruptly bent towards the upper surface of the
mouth ; its anterior margin is divided into eleven sharp-pointed,
recurved, minute, dental processes. The posterior and inferior dental
plate may be' said to consist of two similar but smaller semilunar
pieces, with the mesial margins approximated and conjoined; the
number of dentations on each of the lateral moieties of this lingual
armature, is seven.
The mode of development and reproduction of all these teeth
is the same both in the myxinoids and lampreys. The matrix: is
persistent, as in most other horny productions, and the new conical
tooth is developed immediately within and beneath the base of the
old one ; the same secreting surface which formed the one, produces
(1) PI. 2, fig. 5, a. (21 ib. b.
the other. A vertical section of any of the teeth of the lamprey
displays one or two cones of reserve, between the tooth in use,
and the surface of the matrix ;(1) and the outermost is in general readily
displaced; often, indeed, difficult to preserve in situ in the preparation.
Thin transverse sections of the teeth of the lamprey, viewed
by a magnifying power of a quarter of an inch focus, (2) exhibit their
structure as composed of closely-aggregated parallel tubes, placed
perpendicular to the secreting surface, and having a diameter of
of an inch.
In chemical composition, the teeth of the Cyclostomes resemble
horn.
C H A P T E R III.
T E E T H OF P L A G I O S TOMES .
SHARKS, OR SQUALOIDS.
12. All the genera of true or fixed-gifled plagiostomes, save
Pristis,a.re characterised by numerous teeth, which are restricted in their
situation to the upper and lower jaws. Here, they are arranged in a
greater or smaller number of rows, which succeed each other from
behind forwards, and are attached only to the mucous and fibrous
membranes covering the maxillary cartilages.
Before entering upon the consideration of the teeth, a few words
seem necessary respecting the analogies of these dentigerous cartilages
or jaws. In the common skate, they are in the form of
simple arches, each arch consisting of a pair of cartilages joined
together by a ligamentous symphysis at one extremity, and suspended
by the opposite end through the medium of a common cartilaginous
pedicle from the sides of the cranium. This pedicle is
obviously the homologue of that to which the lower jaw is suspended
in the higher oviparous vertebrates, and which includes more or fewer
(1) ' Pi. 2, fig. 6.
(2) The instrument employed in these observations, and referred to throughout the present
work, is the compound achromatic microscope of Ross.