tinct species which present the same intimate structure of the dental
substance.
In the Chimera monstrosa, the dental plates differ considerably in
their general form and disposition from those of the Chimera or Callo-
rhynchus australis; they are extended in the vertical more than in the
horizontal direction, sheath the exterior of the jaws, and serve as instruments
for cutting and dividing rather than for triturating and
crushing. The anterior teeth of the upper jaw, more especially, present
characters of which we find no trace in the teeth of the last
described recent species ; the dental substance being so arranged as to
resemble a series of oblique superimposed lamellse. The fossil dental
plates of the extinct species of Chimera, discovered and described by
Dr. Buckland, resemble the northern rather than the southern Chimaera
of the present day ; some of these teeth, as those of Chimera Colei, and
Chim. Owenii, resemble in size, though they differ in form from the
dental plates of Chimera monstrosa, but the dental plate of the lower
jaw of the Chimera Townsendii, which is upwards of six inches in
length, indicates a species which, ceteris paribus, must have surpassed
eight feet in length.
In the marine eocene formation called the London clay, at Sheppey,
the dental plates and portions of the jaws of an extinct Chimera, three
or four times the size of the existing species, are occasionally found;
the laminae of the anterior tooth of the upper jaw are bent obliquely,
so that the outer margin of the plate presents a series of lines, which
are horizontal on its anterior part, and bent obliquely upwards at the
side. The posterior dental plate has its outer wall composed of a
series of vertical columnar portions, the lower ends of which, as they
are worn by attrition, cause the trenchant margin of the jaw to be
notched and irregular. The inner surface of the same plate is
convex, smooth and punctate ; it encloses, as it were, a portion of the
jaw itself.
In the extinct Chimseroid subgenera Edaphodon and Passalodon,
discovered and so called by Dr. Buckland, the teeth are long,
broad, and thick, with surfaces adapted for crushing and bruising,
like the dental plates of Callorhynchus; but, instead of encasing the
alveolar borders of the jaws, they are themselves inclosed in the substance
of the jaw, which presents the coarse cellular structure of the
bone of osseous fishes. The teeth are not, however, loosely implanted
in these alveolar cavities, but wherever they are in contact with the
bone, they are anchylosed thereto. Their structure is, nevertheless,
strikingly different from that of the jaw itself, and corresponds with
the modification of the tubular structure presented by the teeth of the
Chimera. In the Edaphodon there are three of these large teeth on
each side of both jaws : their position is nearly horizontal and their
grinding, surface is pitted with small impressions.(1)
In the tooth of the Chimaera, the complex structure arising, as it
were, out of an aggregation of long slender and simple cylindrical teeth
is most conspicuous. Each of the parallel medullary or vascular
canals, which from their large size are easily distinguishable by the
naked eye, is the centre of radiation to very numerous and closely
compacted calcigerous tubes, having a diameter of, ,•1vsth °f an at
their origin, but ramifying and diminishing in size as they recede,
and ultimately terminating in a minute irregular cellular ossification
which occupies the interspaces of the different systems of radiated
tubes, and cements them together in a continuous mass in the recent
dental plate. The course of the calcigerous tubes is at right angles to
the medullary canal, and they have a wavy disposition ; their primary
branches are sent off at an acute angle, and the fine ramuli from these
branches dilate or terminate in minute cells considerably smaller than
the radiated cells of mammiferous bone.
In the fossil teeth of the large extinct Chimere, the coarser cellular
structure, which cements together the systems of calcigerous
tubes, is generally more or less decomposed, while the denser parietes
of the medullary canals formed by the calcigerous tubes remain ; the
coarse tubular structure of the tooth thus displayed, is accurately
figured by M. Agassiz in the Chimera Townsendii. But the medullary
canals and their radiated systems of calcigerous tubes are not only
cemented together by the ossified capsules, but are still more effectually
connected together by the frequent anastomoses of the parallel medullary
canals themselves. The portion of the tooth figured (PI. 29, fig. 2)
exhibits some of these anastomoses. In those parts of the section in
(1) Proceedings of the Geological Society, 1838, p. 687.
v 2
r J