tooth. The unobliterated area of the medullary canals was occupied
by a vascular pulp in the living animal, and the silicious matter with
which they are filled in the fossil, has received a dark stain, probably
from the colouring matter of the vascular pulp ; hut the finer tubes,
from the want of this difference of colour, are in many parts obscurely
visible, if at all. They are discernible in some situations
crossing the concentric lamellae at right angles to the central canal.
The chief difference between the appearance presented by the Haversian
canals of the tooth of Acrodus, and those in bone, is in the
absence of the radiated cells or corpuscles. At the base of the
tooth, there are cells interspersed with the medullary canals, irregular
in size and form, very minute, and appearing like simple
granules without radiating lines. The character of the main or
coarser canals and calcigerous tubes of the ivory of the tooth of
Acrodus, reposes on their undulating course, their rapid diminution
and branching, and the moderately acute angles at which the branches
are given off, except at the circumference of the tooth, where they run
nearly parallel to each other. The line of demarcation between the
coarser and finer ivory, is formed by a series of small cells having the
form and arrangement represented in PI. 16, fig. 3, in which many of the
finer branches of the coarse ivory terminate, and from which the
minute tubes of the enamel-like ivory commence. The superficies
of the tooth is slightly punctated; the depressions, however, do not
correspond with the mouths of tubes, but with the interspaces of
whole groups of the coarser tubes.
20. Hybodus.—The remains of the extinct genus of Plagiostomes to
which M. Agassiz has given the name of Hybodus occur in the
secondary formations from the upper red sandstone to the chalk inclusive.
They consist of teeth and large osseous spines, which have
been discovered so associated together as to leave little doubt that
the recent animal was armed with a pair of spines, one to each dorsal
fin, as in the Cestracion, and that the jaws were beset with closely
packed vertical rows of teeth, containing from six to seven in each
row, (PI. 11, fig. 1). The teeth are in the form of transversely elongated
depressed cones, and consist of two pretty equal parts, viz : a
coarse, osseous, wrinkled base, and an enamelled crown. The crown
is separated from the base by a slight constriction, above which it
swells out and then quickly diminishes to an apical ridge, which
traverses its long diameter, and presents a series of pointed cusps ;
of these, the middle one is the largest and highest; the rest quickly
diminish in size as they recede from it. The sides of the crown are
traversed by well marked but fine ridges, which converge from below
towards the middle cusp, but are variously disposed in the different
species of Hybodus.
The cusps of the exterior teeth are erect, those of the internal or
posterior ones are recumbent, but the transition from one position
to the other is gradual and progressive as in Squatina and the anterior
teeth of Cestracion. The development of these teeth corresponds
with that in the Cestracion, where the whole breadth of the pulp is
progressively ossified from above downwards, instead of from the
external surface of the whole crown inwards ; hence, the last formed
teeth are not hollow, as in the true sharks.
The teeth of some species of Acrodus, as the Acrodus minimus
of Agassiz, resemble so closely those of a Hybodus in external form,
as to be liable to be mistaken for those of that genus. The microscopic
structure of the teeth, in these closely allied genera, is very
similar in its general plan. In Hybodus, however, the calcigerous tubes
in the body of the tooth have a less wavy disposition than in Acrodus.
21. Ptychodus.—The fossil teeth, on which M. Agassiz has established
the genus Ptychodus, have not as yet been discovered undisturbed in
their natural position in the jaws, so as to demonstrate the affinity
of the extinct fish to the recent Plagiostomes, with the same certainty
as in the case of the Hybodus and Acrodus; nevertheless, their
number, their external form, and the absence of any other parts of
the skeleton in the localities where they are most abundant, alike
bespeak that they belonged to a cartilaginous fish, which M. Agassiz
conjectures to have been nearly allied to the Cestracion. The microscopic
texture of these interesting fossils, which 1 described at the
meeting of the British Association in 1838, afforded the demonstration
finally required of their true nature. The entire tooth (PI. 17,