size towards the anterior portion of the jaw ; the posterior teeth,
which are slightly compressed at their base in the antero-posterior
direction, present at that part about a line and a half in diameter ;
the anterior ones measure half a line across the same part ; the length of
some of these small anterior teeth above the sockets is three lines, they
are terminated by a sub-acute apex, 3 he sockets do not he between
parallel lines ; the alternate ones are placed a little more internally.
The teeth were present chiefly in the more external sockets ; hut
where they remained in both, the row of teeth presented the same
slightly zig-zag disposition. The sockets of the teeth are more
shallow in the present than in the preceding fossil ; the outer alveolar
wall is rather more developed than the inner, and the anchylosed bases
of the teeth more nearly resemble in their oblique position those of the
existing Batrachians. The teeth are directed slightly inwards, and are
probably received, like those of the Menopome, into the interval
between the maxillary and vomerine series of the upper jaw, when the
mouth is closed.
The fine external striation and fluting is confined to the basal
third of the tooth, as are also the labyrinthic inflections of the
external cement. Above this part, the dentine consists of fine
calcigerous tubes, radiating from the linear remains of the pulp-
cavity at right angles to the surface of the tooth; being parallel
with the axis of the tooth, where they form its apex, and gradually
inclining outwards until they become transverse to that
axis, which is their general disposition in the body of the tooth,
between the apex and the commencement of the inflected vertical
folds of the cement. At its apical part, therefore, the tooth
of the Labyrinthodon resembles in the simplicity of its intimate
structure that of the entire tooth of the ordinary Batrachia and
of most reptiles. The vertical inflected processes of the cement are
at first short and straight, occurring at pretty regular distances
around the circumference of the tooth ; so that here the tooth partakes
of the structure which has already been said to characterize
the base of the tooth of the Ichthyosaurus. Soon, however, the
primitive inflected folds of cement sink deeper into the dentine and
commence their undulating course, and as the tooth expands,
other processes, at first simple like the preceding, begin to penetrate
the dentine at the interspaces of the primary folds ; these
likewise take on a sinuous course a little nearer the base of the
tooth ; the diverging plates of dentine send off lateral lobes or processes
corresponding with these sinuosities, and a transverse section
at this part of the tooth exhibits the modification of the labyrinthic
structure exhibited in PI. 63 b , fig 2.
The anchylosis of the base of the teeth to distinct and shallow
sockets is a structure in which the Labyrinthodon resembles certain
fishes, as the Sphyrena ; I am disposed also to believe, from the
absence of any excavation in the base of the fixed teeth, or of any
trace of alveoli of reserve for the successional teeth, that these were
developed, as in many fishes, in the soft mucous membrane or gum
which covered the alveolar margin, and that they subsequently became
fixed to the bone by anchylosis. Thus, notwithstanding the close
resemblance to the Crocodilian type which the Labyrinthodon presents
in the form and superficial sculpturing of the skull, it deviates
widely in the mode of fixation and reproduction of the teeth.
Nor is it extraordinary that the present extinct Batrachian genus
should have its relations of atfinity thus radiating in different directions,
since we find in the extinct reptilian forms of a later epoch
the combination of Saurian characters with Ichthyic vertebræ and
extremities.
89. Labyrinthodon pachygnatlms.—A second and larger species of
Labyrinthodon is established upon certain fossils from the lower sandstones
of Warwick, of which I shall here describe those that appertain
to the dental system. The most instructive specimen is a well preserved
and considerable proportion of the right ramus of the lower
jaw, measuring nine inches and a half in length ; the anterior extremity
of this specimen is represented of the natural size at Plate 63 a ,
fig. 4 and 5. The bone is thick and rounded, one inch and a half
deep, and one inch broad at the posterior fractured part, thirteen
lines broad and eight lines deep at the anterior expanded and subdepressed
end, which is curved inwards towards the symphysis of the
jaw, and which supports two of the great cuspidate or laniary
teeth, and the socket of a third. The structure of this lower jaw,