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382 DENTAL ANAT03IY. —;
tortuous. The arrangement of the dentine is as follows : The axial
portion of the tooth consists of a central cone of dentine hollowed out
in the centre to receive the pulp. In cross-section this cone appears as
a ring surrounding the pulp-cavity ; from it plates of dentine, which are
cleft by the fissures from without, radiate to the periphery, pursuing the
same tortuous coarse as that of the fissures. These dentinal plates are
separated from each other by fissures which radiate from the axial cav-
ity, but do not reach the exterior of the crown. Some of the dentinal
plates do not arise from the central ring, but appear on transverse sec-
tion as processes from the periphery of the crown directed toward the
central axis, thus causing the fissures which radiate from the pulp-cavity
to become bifurcated at their outer or peripheral terminations. Some
of these accessory processes reach but a short distance toward the inte-
rior, while others penetrate halfway or more to the centre.
This complexity in the arrangement of the tooth-substances has sug-
gested the name of the typical genus, Labyrmthodon, which was orig-
inally described by the great anatomist Prof. Owen. Just how it has
been produced is difficult to understand.
Teeth of the Reptilia.—This class of vertebrated animals in-
cludes snakes, lizards, crocodiles, turtles, etc., and, considering the
extinct as well as the recent forms, is divisible into eleven distinct
orders, according to Prof. Cope's classification. Of these, but five are
represented in the existing fauna, the others having become extinct in
the different epochs of the earth's 'history.
The batrachians make the nearest approach to the permanent gill-
breathing vertebrates in all the essential features of their structure ; the
Reptilia, on the other hand, furnish us with the transitional forms lead-
ing to the avian and mammalian stems. It is a very significant fact
and one upon Avhich the doctrine of evolution is primarily based, so far
at least as the Vertcbrata are concerned—that the lowest forms appeared
first in the order of time, and were followed by those higher in the
organic scale ; thus we have the cartilaginous fishes as the earliest rep-
resentatives of vertebrated animals; after them come the batrachians
next the reptiles and birds, and finally mammals. It must be borne
in mind, however, that the highest of one group is not always most
nearly related to the group next above it ; for example, if Me compare
tlie structure of a bony fish with that of a salamander, a great interval
will be found to exist, but if we institute a comparison betMcen the latter
and a dipnoan fish, which is comparatively little removed from the car-
tilaginous forms, this interval will be found to be materially diminished.
Thus, the conclusion is obvious that the Batrachia sprang not from the
higher bony fish, but from some generalized representative of the pis-
cine type. The same reasoning can be applied to other divisions.
As regards the Keptilia, the chief distinction between them and the
Batrachia consists in the circumstance that the latter during the larval
stages of their existence breathe by means of gills like the fishes, Avhere-
as the Reptilia breathe by means of true lungs from the time of birth.
Important osteological differences are found in the bones of the skull.
The earliest ap])earance of the Reptilia dates back to the Permian
Epoch, v.here they are represented by a group of peculiar batrachian-