grinding surface, with a great breadth of crown. It is of one of those
teeth of the typical Psammodi which I have submitted to microscopic
examination. A transverse section of the tooth of this genus presents
the appearance, under a moderate magnifying power, as if it
were composed of close-set coarse tubes, the arese of which were thus
exposed. Such a section, viewed with a power of 400 diameters,
shows that these tubes are surrounded by concentric lamellae, like the
Haversian canals; and that these lamellae, and the clear interspace,
which is generally equal to the thickness of the lamellae, are permeated
by minute irregularly disposed tubes, which anastomose in the clear
interspace, and open into extremely minute cells, scattered through
that part. A longitudinal section of the same tooth shows, in many
places, the whole course of the canals; they run nearly perpendicularly
to the convex superficies of the tooth, and, consequently, incline
outwards at-the sides of the section. They lie nearly parallel with
each other, with interspaces equal to from six to eight times the
diameter of their area, and branch dichotomously once or twice in
their course. Each canal is surrounded by concentric layers of
a dark colour, encroaching upon one-third of the interspace, which
thus presents two semi-opake streaks and one intermediate clear
line; the whole of these interspaces is perforated by the irregular
wavy, branched, anastomosing calcigerous tubes. The terminations
of the medullary canals near the periphery of the tooth are slightly dilated,
and give off in every direction calcigerous tubes corresponding to
those in the interspaces of the canals. The calcigerous tubes, at this part
of the tooth, run obliquely along the side of the medullary canal, from
which they are continued, for a short distance before they pass off
through the concentric layers into the clear interspace; to this
structure is due the appearance, which the medullary canal presents,
of being composed towards its termination of a fasciculus of spirally
twisted tubes. I have not observed this structure in any part of the
medullary canals of the tooth of the Ptychodus, which further differs
from that of Psammodus in the more frequent bifurcation of the medullary
canals, and in the smaller extent of their opake concentrically-
laminated walls. The structure of the tooth of Psammodus, like that of
ptychodus, differs from that of Acrodus- in the greater number and
more parallel course of the medullary canals, their fewer branches,
and in the absence of an external layer of fine parallel tubes.
The Psammodontoid teeth which are elongated, more or less
contracted and truncated at the two extremities, and of which the
surface of the crown is reticulated, are now regarded by M. Agassiz
as indicating a distinct subgenus, to which he gives the name of
Strophodus, (PI. 17, fig. 4). Other fossil teeth, which resemble in
general structure those of Psammodus, but which have the centre of
the crown elevated into an obtuse transverse cone, and traversed by a
ridge, from which oblique furrows diverge, but with a more or less
transverse direction, towards the circumference and there ramify,
have been referred by the same authority to a genus called Orodus.
These fossils occur in the older secondary rocks.
Similar fossil teeth, having the crown in the form of an elevated
obtuse cone, but perfectly smooth on the surface, form the type of the
i genus Helodus. Agas.
In a fourth form, the crown is compressed and elevated, and
sometimes terminates in a sharp edge like the tooth of a Carchanas;
its base is always surrounded by a series of concentric folds. This
is the type of the genus Chomatodus. Agas.
A fifth forifi of Psammodontoid teeth, in which the crown is
raised, subcompressed and subdivided by deep transverse ridges into
dentations, varying as to number and degree of sharpness or obtuseness,
has given rise to the establishment of the genus Ctenoptychius,
Agas.
23. Petalodus.—In the teeth of manyof the subgeneraof Psammodus,
the crown is produced into a median or submedian ridge ; if the body
of the tooth be supposed to be still more compressed, so that the
ridge should terminate the contour of the crown like a trenchant
edge, there will then be produced the lamelliform figure which characterizes
the teeth of the subgenus in question.
In the specimen of the tooth of Petalodus Hastingsii now before me,
which I owe to the kindness of Sir Philip Egerton, the trenchant margin
is slightly convex andfinely serrated, the crown of the tooth is invested with
a thin layer of dense enamel, with a smooth and shining surface, the