the Plagiostomous order, among fishes, to include so many genera
which exhibit in the dental substance the rich organization of vascular
and calcigerous tubes that has already been described, and the
modifications of which become the more important and interesting,
when, as in the case of the extinct Psammodus, Acrodus, and Hybodus,
they are almost the sole features of the organization of those most
ancient vertebrate animals which remain for the contemplation of
the anatomist and physiologist.
CHIMÆROIDS.
25. I next proceed to consider a well marked modification of the
same highly organized dental structure in the genus Chimcera, which,
like the Cestracion, is an extreme but still more anomalous modification
of the chondropterygious type, having the branchiæ unattached
externally : this genus is also the representative of several extinct forms
of fishes.
The jaws of the Chimæroid fishes are armed, says Cuvier, in the
characters assigned to this family in the Règne Animal, (1) with hard
and indivisible plates instead of teeth, four above and two below, and
in the 2nd edition of the Leçons d’Anatomie Comparée, (2) these
dental plates are described as being salient, trenchant and striated.
They have not, as yet, been more particularly described, but the
coarse medullary tubes which enter into their composition, and to
which Cuvier alludes when he compares the structure of these teeth
with those of the Orycteropus in his Histoire Naturelle des Poissons,
(3) are illustrated in PI. 40, fig. 20, of the great work on Fossil
Fishes by M. Agassiz.
The teeth of the Callorhynchus or Chimcera Australis, are
represented as they appear on looking into the widely opened
mouth in Plate 28, fig. 1 . The upper jaw presents two anterior
dental plates, of a small size and semi-elliptic form, and two posterior
(1) Tom. ii, p. 382.
(2) Tom. iv, p. 362.
(3) Tom. i, p. 496. Cuvier’s words are, “ Leur tissu intérieur est percé de tubes fins,
comme un jonc ou comme les dents de l’orycterope.” The tubes here mentioned are the medullary
or vascular canals, and are merely the centres from which the true calcigerous tubes
radiate.—See Report of British Association, 1838, p. 145.
I triangular plates, six times larger than the preceding. The anterior
I angles of these plates are rounded, and conceal the posterior half of
I the anterior plates : the inferior broad surface of the anterior teeth is
I sinuous, and joins the lateral surface at a sharp edge; the grinding
1 surface of the posterior plates is convex in every direction, and there I is a raised portion in the middle of each, of the figure represented in
I the plate (PI. 28, fig. 1). The upper surface of each of these teeth is I concave from side to side, so that it encases or sheaths the alveolar
I border of the upper jaw, in a manner analogous to the broad teeth of
I the Cestracion.(\) Both the anterior and posterior dental plates in the I upper jaw, meet at the median line of the mouth. The two dental
I plates of the lower jaw are of a subtriangular form, with the posterior
I and external sides gently curved; the broad grinding surface is con- I vex on the inner and concave on the outer side ; a trenchant margin
I divides this from the lateral surfaces of the dental plate. When a
I longitudinal vertical section is made of these teeth (as in fig. 3 , pi. 28),
I their coarse tubular texture is evident to the naked eye. There is a
I large pulp-cavity at the posterior parts of both the upper and lower
■ dental plates, and, when the pulp is removed, the exposed surface of
I the base of the tooth presents a reticulate character from the large areae
I of the medullary tubes into which the processes of the pulp are con-
Itinued. These tubes radiate towards the grinding surface of the tooth
land dichotomize as they proceed. As these tubes advance towards
■ the surface, their cavity becomes gradually obliterated by calcareous
I salts, deposited in concentric layers and perforated every where by the
■ minute calcigerous tubes which radiate from the medullary canal ;
I thus the substance of the tooth increases in density as it approaches
■ the triturating surface.
A diminished view of the appearance of a longitudinal section of the
■ dental plate of a Chimcera, as seen under a magnifying power of 400
■ diameters, is given at PI. 29, fig. 1, and of a transverse section at fig. 2 .
I But before proceeding to describe this structure, a few words are requi-
Isite touching the modifications of form presented by the dental plates
Ipf Chimcera monstrosa, (PL 28, figs. 4 and 8), and of some allied ex-
I (1) The genus Cochliodus appears to have been an extinct transitional form between
■ ^frucion and Callorhynchus.
F