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TEETH OF THE VERTEBBATA. 381

the evagination of the lining membrane of the oral cavity. Whether
this is caused by unequal rate of growth of the surrounding parts,
whereby this lining membrane is forcibly pulled outward and replaced
from within, or whether the formative energy inherent in the membrane
itself causes it, is difficult to determine. This primary cause of succes-
sion is profoundly affected by the development of ait osseous sheath
from this same membrane around the central cartilaginous axis of the
jaws. It seems plausible that secondarily the cause of tooth-succession
is to be sought for in the proliferation of the cellular elements beneath
the young and growing germ.
To the inner side of the external osseous wall of this groove in Meno-
poma the functional teeth are attached ; their bases are slightly enlarged
and extend quite to the bottom of the groove, while the tapering crowns
reach considerably above the level of the jaw. Attachment takes place
by the ankylosis of that part of the base which is in contact with the
outer wall to the bone of the jaw through the intermediation of osteo-
dentine. This manner of implantation is known as " pleurodont," on
account of the fancied resemblance of the teeth so attached to ribs.
The teeth in use at any given time are from thirty to forty in num-
ber upon either side in each jaw ; they are subequal in size, and are
placed with great regularity, being separated by spaces about equal to
the width of a single tooth. Their crowns are sharp-pointed and
slightly recurved ; they are said in some species to be tipped with
enamel, which is probably true of all. The vomerine teeth are fewer
in number than the maxillary, there being not more than twelve or
fifteen upon either side. Their line of direction and manner of implan-
tation coincide with the maxillary row external to them, agreeing with
those also as to size and form.
In some of the extinct batrachians, notably the labyrinthodonts, there
were several teeth of the maxillary and premaxillary set considerably
enlarged and of a caniniform pattern. The species of this section were
mostly of large size and presented a formidable dental armature. They
likewise differ from all other of the Batrachia in that the teeth were
implanted in distinct sockets, and were rarely if ever attached to the
body of the jaw by ankylosis. The structure of the teeth is a curiously
complex one, and finds no parallel throughout the entire Yertel^rata,
save in one extinct saurian (Ichthyosaurus) and several fishes, which
exhibit a similar condition in a less perfected degree.
The external surface of the crowms of the teeth in the labyrintho-
donts is marked by a number of longitudinal ridges, separated by what
at first sight Mould appear to be comparatively shallow grooves extend-
ing from the base to the apex of the crown. Upon cross-section, how-
ever, these fissures are seen to penetrate into the body of the tooth to a
remarkable depth—to a point, in fact, quite near the pulp-cavity or cen-
tral axis of the tooth, where they are separated from it by a thin wall
of dentine. The entire outer surface is covered by a thin layer of
cement, which is reflected inward to the bottoms or internal termina-
tions of the fissures just mentioned. The cut edges of this reflected
layer of cement, which is of uniform thickness throughout, are almost
straight for a short distance beneath the surface, but soon become very