which was broken across into two nearly equal portions, was the
first object of attention. On the inner side of the anterior moiety
it appeared to he strengthened by an opercular piece in the form
of a thin plate, gradually narrowing to a point, and terminating
at the beginning of the inward curvature of the ramus (fig. 4).
Two transverse fractures of this lower jaw display the relations
of the external plate of the angular piece with the thin internal
bony lamina ; the two pieces are uninterruptedly confluent, and form
a single broad and strong piece of bone, supporting the dentary piece
upon a groove along its upper surface, and terminating anteriorly
at the bend of the expanded premandibular or dentary element, which
there receives the extremity of the angular hone in a notch ; the
hatrachian character is thus as clearly established by the present
as by the preceding fragment of the lower jaw. A similar portion
of the lower jaw of a Saurian would have exhibited either the dentary
element alone, or the anterior extremity of the opercular element in
the form of a thin plate applied and limited to the inner side of the
dentary piece. The continuation of the angular element alone, forming
the lower half of the ramus, to near the symphysis, and supporting
the dentary piece in a groove on its upper surface is as striking a
Batrachian character in the fossil of the British sandstone, as that
observed by Prof. Jaeger in the occipital hone of the great Labyrin-
thodon (Salamandroides giganteus) Jaegeri of the German Keuper.
The smaller serial teeth in the present portion of jaw are about
forty in number, and their sockets are in close contact with each
other; they very gradually diminish in size as they approach both
ends of the series. Two of the smallest teeth (&') at the anterior part of
the jaw are recumbent, in front of the great laniaries ; they mayhave
been incompletely developed teeth of replacement, not yet erected and
anchylosed to the bone.
The alternate sockets are empty in a considerable portion of the
posterior part of the series, agreeably with the order of shedding
and renovation illustrated in the Labyrinthodon leptognathus, so that
the teeth thus appear to be separated by wider intervals than they
really are. The form of the teeth is conical, with the base slightly
compressed in the direction of the axis of the jaw ; the largest transverse
diameter of one of the posterior of these serial teeth, where it
emerges from the socket, is three lines ; the same diameter of the
anterior serial tooth is one line and a half; its length, four lines and a
half. The great laniary teeth (a) appear to be three in number in each
symphysis, and the one nearest the symphysis is somewhat larger
than the others, hut they are probably not all in place and use at the
same time. The greatest diameter of the sub-compressed base of the
largest of these fangs is five lines; its length, judging from an entire
tooth of the same species, must have been at least one inch and a
half. The lines of the inflected cement form well marked longitudinal
strise around the basal half of the tooth, and the interspaces
of the strise form convex ridges, as in the large tooth, the labyrinthic
structure of which was described at p. 201. These ridges are
fewest near the termination of the strise, being divided and multiplied
by new longitudinal strise, caused by new inflected folds of the
cement near the basis of the tooth ; the apical half of the tooth has
a smooth and polished external surface ; the pulp-cavity is continued
of small size into the centre of this part of the tooth. In the
serial teeth, which, except their less gradual diminution of size, correspond
with the anterior larger tusks, the central pulp-cavity is more
quickly obliterated; the texture of the teeth is dense and brittle.
I have examined the structure of the serial teeth in a small
fragment of the upper jaw of the Labyrinthodon pachygnathus,
from the same locality as the lower jaw; this fragment was
three inches and a half in length, and included twenty-four sockets of
the serial teeth, the alternate teeth being in place though broken.
These teeth precisely corresponded in size and form with those of the
lower jaw. The labyrinthic structure is confined to the basal half of
the tooth, where it is indicated by the external striation, and becomes
more complex as it approaches the base. The blending of the external
layer of cement with the dentine, as exhibited by a transverse section
of the tooth above this part, well illustrates the principle of the
more complicated modifications of the labyrinthic structure first discovered.
The processes of dentine which radiate from the pulp-cavity
(PI. 63 b , fig. 1, a), are twelve in number at the line of the section
here described, but most of them divide in their course outwards ;
the corresponding pulp-fissures diverge in straight lines, bifurcate