the central or corn-stone division of the Old Red Sandstone. The three
specimens here described are from Scat-crag, near Elgin. The largest of
these is represented of the natural size at PI. 62 a , fig. 1. It is conical,
subcompressed, subincurved, with a round obtuse summit (a), and a
sub-circular base, the margin of which, in the specimen, was
rounded off or bent in, and the surface of the base was slightly
depressed in the centre, and rough as if it had been detached from an
anchylosed union with a shallow alveolar depression. On each side
of the tooth, a little nearer the convex than the concave surface,
there is a well marked longitudinal ridge, which gradually subsides a
little below the summit. The position of these two ridges, and the
degree of compression of the tooth at its middle part, are shown in
the outline of the transverse section of the tooth by the side of the
figure. The smooth surface of the tooth is traversed by very fine
longitudinal linear impressions extending from the base to within
one fourth of the rounded apex.
A transverse section taken one line above the base of this tooth,
exhibited a dark irregularly dotted central part, and a lighter peripheral
structure: it is represented at PI. 62 b , fig. 1. A portion of
the same section corresponding with that intercepted by the two lines
in fig. 1 , as viewed by transmitted light with a magnifying power of
fifty linear dimensions, is represented on a reduced scale at fig. 2 .
Thus magnified, a central pulp-cavity of relatively small size and
of an irregular lobulated form is discerned, a portion of which is
shown at a ; this is immediately surrounded by the transverse sections
of large cylindrical medullary or pulp-canals of different sizes ; and,
beyond these, there are smaller and more numerous medullary canals,
which are processes of the central pulp-cavity. In the transverse
section these processes are seen to he connected together by a network
of smaller medullary canals belonging to a coarse osseous
texture into which the pulp has been converted, and this structure
occupies the middle half of the section. All the medullary canals were
filled by the opaque matrix. From the circumference of the central
net-work, straight medullary canals radiate at pretty regular intervals
to the periphery of the tooth ; most of these canals divide once, rarely
twice, in their course; the division taking place sometimes at their
origin, in others at different distances from their termination, and
the branches diverge slightly as they proceed. Each of the above
medullary rays is continued from a short process of the central structure,
which is connected by a concave line with the adjoining process,
so that the whole periphery of the transverse section of the central
coarse reticulo-medullary body of the tooth presents a crenate outline.
From each ray and its primary dichotomous divisions, short branches
are sent off at brief intervals, generally at right angles with the trunk,
or slightly inclined towards the periphery of the tooth. These subdivide
into a few short ramifications like the branches of a shrub,
and terminate in irregular and somewhat angular dilatations, simulating
leaves, but which resolve themselves into radiating fasciculi of
calcigerous tubes. There are from fifteen to twenty-five or thirty-six
of these short and small lateral branches on each side of the medullary
rays.
In a section of the same tooth, one third from its obtuse summit
(PI. 62 b , fig. 3), the irregular central pulp-cavity was lost, and in its
place there were a few large medullary canals connected by a fine network
of smaller canals. This tissue occupied rather more than one third
of the diameter of the section. From its periphery there was continued
the same system of nearly straight, sparingly dichotomizing, medullary
canals, radiating at regular distances from the central tissue to the
periphery of the tooth. The radiating canals here also give off short
lateral branches, but these do not terminate in such well marked
dilatations as those observable at the base of the tooth : the transverse
branches here generally spring from short alternate lateral
bendings of the main stem. There are about forty radiating medullary
canals in this part of the tooth, and about fifty in the section
taken from the base of the tooth.
In both sections the angular dilated terminations of the small
lateral branches of the medullary rays form, as has been said, the
centres of radiation of as many systems of the most minute calcigerous
tubes, each system constituting a lobe of the dentine, separated
from the adjoining lobes by an extremely thin layer of cement. In
the section nearer the apex of the tooth, the radiating systems of
calcigerous tubes, forming similar lobes of dentine are given off