its base to its sub-acute apex. The surface of the tooth is much
smoother thau in the Dondvodus stviyotus. I took a transverse section
from the base of this tooth, and finding the same essential radiated
disposition and dendritic ramifications of the medullary canals, I
made a longitudinal section of the rest of the tooth.
In this section eight principal medullary tubes were seen ; the
four largest and closest together were situated in the centre; the others
diminishing as they approached the side. The central canals run parallel
to each other and to the axis of the tooth along the basal half;
the two middle canals so continuing to the apex. The small lateral
tubes bend outwards to the side of the tooth; the third on each side
sends olF one or two primary branches in the same oblique direction
and also finally terminates by bending towards the margin of the
tooth. The primary divisions of the large medullary tubes run parallel,
or nearly so, to the trunks, but begin slightly to diverge as they
approach the apex. The primary divisions of the small lateral tubes
are given off at different, but always acute angles. The smaller or
secondary branches are given off frequently at right angles, and those
from the primary branches sometimes retrograde to the main tubes.
The plan of branching of the whole system of medullary canals which
pervades the. entire substance of the tooth is strikingly similar to that
of certain forest trees, as the elm, but with the characteristic difference,
which distinguishes in this, as in most other cases, the branching
of an animal from a vegetable structure, namely, that the terminal
branches anastomose, forming a coarse but elegant net-work,
occupying with considerable regularity the whole of the interspaces
of the longitudinal medullary tubes.
A transverse section of this tooth exhibits three-fourths of its
central part occupied by a reticulo-medullary structure; and the
peripheral portion traversed by medullary rays. In the central
structure are seen the arese of numerous, nearly equal sized and
equi-distant medullary canals, around which the dentine is disposed
in well-marked concentric layers, and from which the calcigerous
tubes radiate as in the Myliobate and Orycterope; but these tubes
are relatively fewer and less parallel in their course., and form more
decided reticulations in that course. The shape of each component
denticle, thus seen in transverse section, is more rounded and less
regular than in either of the two above-cited instances. The cellular
cement which separates each denticle is relatively thicker. The
diameter of the central medullary canal is generally equal to two
thirds the diameter of the laminated and tubular wall of the denticle,
of which it forms the axis. In the peripheral structure, the radiated
trunks are shorter in comparison with the central cellular part than
in the preceding species. They are relatively wider apart from each
other, are less parallel in their course, their primary divisions diverge
at a more open angle, and their lateral branches are longer.
The branches of the radiating canals, after dichotomising, are
resolved at their extremities into radiated systems of calcigerous tubes
terminating in a thin stratum of calcigerous cells. This stratum
forms the boundary of each system and thus describes an undulating
line on each side of the radiated canals, which is very similar to the
anfractuous inflected layer of the external cement in the Labyrin-
thodon. In the transverse section of the Dendrodus hastatus the
calcigerous tubes seem to form, towards their extremities, a fine
net-work with open meshes, and to be fewer and at a greater relative
distance from each other than in the first described species.
It is obvious that the plan of structure, as exhibited in the
longitudinal section of the tooth of the Dendrodus, bears considerable
anaiogy to that in the shark, and Scomberoid fishes, as the Sphy-
rsena, Dictyodus, &c.; but one cannot fail to recognize a greater
amount of parallelism in the medullary tubes in the Dendrodus, and
the systems of calcigerous tubes which diverge from the medullary
canals still more strikingly exhibit the difference which depends upon
their straighter and more parallel course, and which indicates the
higher type of structure in the teeth of the Dendrodus. These tubes,
in fact, instead of forming the inextricable moss-like reticulations,
which occupy the median interspaces of the medullary canals in thé
Shark and Sphyrasna, here run at right angles to the sides of the
tube from which they are continued, and parallel to each other, as
far as the middle of the interspace ; there they meet the extremities of
the opposite series of calcigerous tubes and are lost in minute cells.
The differences above manifested in the dental structure of the
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