more frequently from the sides of the lateral branches, than from
their terminal dilatations.
The peripheral extremities, also, of the medullary rays, and of
such of their subdivisions or branches as are nearest to and directed
towards the margin of the section, and consequently to the periphery
of the tooth, are resolved into fasciculi of calcigerous tubes, which
diverge in graceful curves from their point of origin : two of these
peripheral lobes are figured at PI. 62 b , fig. 4. The middle tubes are
continued from the interspace of the diverging ones to the periphery
of the tooth in a line parallel with that of the main medullary canal
(a), which radiates from the central reticulation 1 the lateral tubes
gradually diverge more and more from this line, and those which
proceed from the sides of the extremity of the medullary canal run
at right angles to its course. The lateral tubes terminate, together
with those of the adjoining medullary canal, in a linear series of calcigerous
cells; which line is continued inwards from the periphery
of the tooth, like a process of the external capsule, inclosing and
defining, as it were, each of these terminal square lobes or systems
of calcigerous tubes. There is a slight indentation of the periphery of
the transverse section at the line of the above described inflection of
the cellular structure ; and this indentation (b) is a section of one of
the fine superficial longitudinal striae already mentioned. The inflected
line of minute cells may be traced, in most parts of both sections,
to near the central reticulate system of medullary canals, sending off
branches on each side, which bound in a similar manner the different
lobes of dentine, or systems of radiated calcigerous tubes which
are given off from the sides of the straight medullary canals. The
external longitudinal fine grooves on the superficies of the tooth indicate,
as above stated, the entering lines of the fine cellular cement, or
the interspaces of the lobes of the dentine appended to the medullary
canals which radiate from the central pulp-cavity or net-work.
The medullary rays, though for the convenience of description
they have been termed canals—as they appear to be in the transverse
section of the tooth, yet are not cylindrical tubes. For it must be
obvious from the similarity of the structure of the two sections here
described from nearly the twTo opposite extremities of the tooth, that
the radiating lines described as medullary canals are, more probably,
sections of vertical fissures, or lamelliform processes of the pulp,
radiating from the central body of the pulp to the periphery of the
tooth, and co-equal with the longitudinal extent of the tooth; but
decreasing in number as the tooth contracts towards its apex.
The species to which the tooth, exhibiting the beautiful and complicated
structure just described, belonged, may be indicated by the
name of Dendrodus biporcatus, in reference to the two opposite ridges
by which the tooth is characterized.
Dendrodus strigatus.—A second and smaller tooth from the same
formation and locality, differs from the preceding in its more elongated
and slender conical form, with a nearly circular transverse section ; it
also differs in the absence of the two longitudinal ridges and in the presence
of broader,deeper, and closer longitudinal impressions separated
by intervening convex ridges. As those characters strongly indicate
a specific difference in the animal to which it belonged, it may be
convenient to attach to this tooth the name of Dendrodus strigatus: its
microscopic structure, as shown by a fine transverse section, proves it
to be generically allied to the Dendrodus biporcatus.
In the Dendrodus strigatus the central system of reticulate medullary
canals occupies a larger proportion of the mass of the tooth.
The radiated canals or processes of the pulp are shorter and more
branched: the systems of calcigerous tubes which diverge from the
numerous processes and branches are separated from each other by a
fine and clear line, and terminate in a broader band of calcigerous cells.
The appearances which the structure of the tooth thus presents in
the transverse section, somewhat resemble those of the tooth of a
Myliobates; but the lobes or systems of calcigerous tubes are less
regular in form and size.
Dendrodus hastatus.-fH-The third tooth presents a more compressed
conical figure than the two preceding, and the opposite ridges
which characterized the Dendrodus biporcatus here form, as it were,
the margins of two cutting surfaces, and the opposite ends of the
flattened elliptical transverse section of the tooth. The length of
this tooth is eight lines, the longest diameter of its base three lines ;
the shortest diameter two lines. The tooth diminishes regularly from