from the prevalence of this structure in the simple conical teeth of reptiles,
I did not build any very strong hopes of detecting such modifications
of dental structure in the similarly simple teeth of the so
called Mastodonsaurus and of the tooth from the Warwick sandstone,
as would be sufficiently marked and obvious to carry conviction of
their generic, much less specific identity. But in this I was agreeably
and unexpectedly deceived.
When I refer to figure 1 in plate 64 a , and state that the first
transparenttransverse section of the tooth of the Labyrinthodon (Mastodonsaurus)
Jaegeri that was placed under the microscope and viewed by
transmitted light, with a low magnifying power, presented the singularly
complicated structure there exhibited, the anatomist, conversant
with the known modifications of dental structure in the animal kingdom,
may well conceive my surprize. It was not, indeed, until I
had had sections made in various directions, from the portions of the
tooth of the Lab : Jaegeri transmitted to me, and had studied them
intently at several successive examinations, comparing the appearances
they presented with those of numerous examples of the teeth of
Saurians, Batrachians and other animals, that I at length comprehended
the nature and principle of the singular cerebriform convolutions
or sinuosities which pervaded every portion of the tooth of this
most remarkable reptile of the Keuper sandstone.
A transverse section from the base of the tooth of the Ichthyosaurus
gave the first clue to the structure of that of the Labyrintho don.
Before investigating the latter I had been accustomed to regard
the tooth of the Ichthysaurus as presenting, at its base, the most
complicated condition of dental structure in the class of reptiles ;
but it is simple as compared with the structure which pervades almost
the entire tooth of the Labyrinthodon Jaegeri.
T° render intelligible the plan of this structure, I may first refer
to that of the base of the tooth of the Ichthyosaurus communis (PI. 64 b ,
fig. 3). Teeth in general, as has been shown, vary in structure according
to the number of substances which enter into their composition,
and according to the disposition of those substances. In the ungulate
mammalia, as the elephant, rhinoceros, horse, etc., in which the
crown of the tooth consists of dentine, enamel and cement, vertical
folds of the enamel and cement penetrate the body of the tooth, and
receive in their interspaces corresponding vertical processes of the
dentine: the consequence of this disposition in maintaining a
grinding surface of the tooth, by the unequal attrition of the edges of
the interblended laminae is well known.
The pattern, however, after which the folds of enamel and cement
are inflected into the substance of the tooth in these and other herbivorous
species, although determinate in and characteristic of each
genus or species, is always more or less irregular and unsymmetrical t
there is no instance in the mammiferous class of these folds converging
at regular intervals all round the circumference of the tooth towards
its centre. Such a disposition of the external substance of the tooth
may be traced at the base of the tooth in a few fishes, but is more
conspicuous in the fang of the tooth of the Ichthyosaur. Here,
the external layer of cement (for the enamel ceases at the base of
the crown) is inflected, at pretty regular distances round the circumference
of the tooth, towards its centre ; the vertical folds being
straight or plane, and extending to a distance about equal to the breadth
of their interspaces. These interspaces are occupied by corresponding
processes of the dentine, which radiate, or diverge from the central
mass of that substance.
If we could suppose the tooth of the Ichthyosaur to be worn
down in the living animal by the masticatory uses to its complicated
basis, then an eighth part of the diameter of the tooth, around its
circumference, would present a series of ridges of the denser substance
converging in straight lines from that circumference.
The plan and principle of the structure of the Labyrinthodon’s
tooth is the same as that of the tooth of the Ichthyosaur but it is
carried out to the highest degree of complication. The converging
folds of the external cement, instead of being arrested at one fourth
of the distance from the circumference to the centre of the tooth, are
continued close to that centre ; and, instead of being simple, straight,
or plane lamellae, they are bent upon themselves in a series of sinuous
folds, resembling the anfractuosities of the brain. The ordinary
laws of the complication of dental structure are here, however,
strictly adhered to, and every space intercepted by a convolution of
the converging folds of the cement, is occupied by a corresponding