process of the diverging layers of the dentine, and thus is produced
the singularly complicated appearance which a transverse section of
the tooth of the Labyrinthodon or Mastodonsaurus exhibits.
The external longitudinal flutings of the base of the tooth of the
Ichthyosaur are much coarser, and more indicative of the converging
vertical folds of the cement, than are the corresponding longitudinal
lines on the exterior of the tooth of the Labyrinthodon ;
which is owing to the layer of the inflected cement being much
thicker in the Ichthyosaur. The external striae of the Labyrinthodon
s tooth are too little conspicuous to attract particular attention,
or to indicate that they are the lines of inflection of a series of
extensive vertical folds of the external substance. Accordingly Professor
Jaeger describes the tooth of the Mastodonsaurus as being longitudinally
striated on the superficies, noting where the stria; terminate
their relative, distances, and where they are most marked: the
texture of the tooth, where it was exposed by fracture, he pronounces,
as indeed it appears to the naked eye, to be uniform, homogeneous
and compact(l), and he concludes his description by stating that the
tooth resembles most closely that of the Lacerta nilotica and of some
species of Monitor.
The teeth, however, of those species of Varanus, Lacerta, Monitor,
and other Saurian genera which I have submitted to microscopic
investigation, have all presented the usual structure of simple Saurian
teeth.
The portion of the tooth of the Labyrinthodon Jaegeri from
which the sections here described, were prepared, included
about the middle third part of a tusk nearly as large as the one
figured by Prof. Jaeger (2). That tooth is three inches and a
half in length, and one inch and a half in breadth at the base,
whence it gradually contracts, with a slight bend, towards the
apex: this is obtuse, with a slightly depressed summit; it is three lines
in diameter, and presents a small rising in the centre of the terminal
depression. The external longitudinal striae are regularly arranged
with intervals of about a line at the base of the tooth; and they maintain
nearly the same relative position throughout the lower three
(i) Jaeger, loc. cit. p. 36. (2) Loc. cit. tab. iv. fig. 4.
fourths of the tooth, by decreasing in number as the tooth diminishes
in thickness ; they finally altogether disappear about half an inch from
the summit of the tooth : and at this part, from the analogy of other
species of Labyrinthodon, it is to be presumed that the structure of the
tooth of the Lab. Jaegeri may be more simple.
The dentine or body of the tooth is invested by only a very thin
layer of cement, and it is a vertical fold or duplicature of this cement
which penetrates the substance of the tooth at each of the striae,
which, as before observed, are arranged at intervals of about one line,
around the whole circumference of the tooth. The inflected fold runs
straight for about half a line, and then becomes wavy, the folds rapidly
increasing in breadth as they recede from the periphery of the tooth ;
the first two, three, or four undulations are simple; then their contour
itself becomes broken by smaller or secondary undulations;
these become stronger as the fold approaches the centre of the
tooth, when it slightly increases in thickness, and finally terminates
by a slight dilatation or loop close to the pulp-cavity, from which the
free margin of the inflected fold of cement is separated by an extremely
thin layer of dentine. The number of the inflected converging
folds of dentine is about fifty at the middle of the crown of the tooth,
but it must be greater at the base. All the inflected folds of cement
at the base of the tooth have probably the same complicated disposition
with increased extent; but, as they approach their termination
towards the upper part of the tooth, they also gradually diminish in
breadth, and consequently penetrate to a less distance into the substance
of the tooth. Hence, in such a section as is delineated, it will be
observed that some of the convoluted folds, as those marked c c PI. 64 a ,
fig 1 , extend near to the centre of the tooth | others, as those marked
d d, reach only about half way to the centre | and those folds, which,
to use a geological expression, are ‘ cropping out’, penetrate to a very
short distance into the dentine, and resemble in their extent and
simplicity the converging folds of cement in the fangs of the tooth of
the Ichthyosaurus.
The disposition of the dentine in the tooth of the Labyrinthodon
Jaegeri is still more complicated than that of the cement. It consists
of a slender, central, conical column, excavated by a conical pulp-cavity