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TEETH OF THE VERTEBRATA. 379

takes place by a movement of the fibrous gum, in which the bases of
the teeth are imbedded, outward over the curved surface of the jaw.
Coincidently, however, witli the ossification of the skeletal axis the
osseous bases of the dermal denticles coalesce to form the dentary bones,
as has been shown by Hertwig. By reason of the development of this
bony envelope of the primitive axis of the jaw, any further movement
of these denticles is prohibited, being firmly co-ossified with it.
During the coalescence of the denticles a portion of the primitive
tooth-bearing membrane is enclosed beneath the fused osseous plates,
and retains its original formative energy, thereby furnishing a source
of supply quite equal to that of the sharks. Tiie denticles whose basal
plates form the sides and under portions of the dentary bones disappear,
while those on the summit of the jaw are retained as teeth. A confor-
mation of this position by evidence other than that afforded by embry-
ology is seen in the dentary bones of Ceratodus, in which each one is
made up of three or four pieces which have failed to coalesce.
The attachment of the teeth, therefore, in this group, as we would be
led to anticipate, is by ankylosis to the dentary and other bones which
support them. Still, there are many brush-like structures which are
identical with true teeth to be found in the mouths of many fishes,
which remain imbedded in the lining membrane and do not develop
any connection with the underlying bones. There are several ways by
which the teeth become fixed to the jaw bones in ankylosis, but the most
common is for the central axis of the tooth to be occupied by a cone of
osteo-dentine, which blends with the bone of the jaw. Several fomilies
of fishes have some of the teeth attached by an elastic hinge, by which
they can be bent down in one direction and resume an erect attitude.*

Teeth of Batrachia and Reptilia.
As we pass from the dental organs of the more typical fishes to those
of frogs, salamanders, newts, etc., constituting the batrachian subdivis-
ion, a marked diminution in the number of individual teeth is to be
observed. With the appearance of perfected air-breathing organs the
complex hyo-branchial skeleton, typical of the fishes, becomes greatly
reduced and simplified as the higher forms are approached ; conse-
quently, the branchial and pharyngeal teeth disappear in all the
Vertebrata above fishes.
In all those Batrachia in which teeth exist they are usually disposed
in a single row on the borders of the jaws, and are supported by the
maxillary, premaxillary, and dentary bones respectively. In addition
to these, each ^•omer (for there are two) bears a single row of teeth,
between which and the maxillary row the lower jaw bites. As a rule,
the Reptilia, on the other hand, lack the vomerine set, but in some of
them (serpents, for example) teeth are developed upon the pterygoids
and palatines, as well as upon the maxillary and dentary bones.
The teeth of the Batrachia present so limited a range of variation that
the description of one will serve to give a general idea of the dentition
of the whole group.
' This fact was first noticed by Prof. Gill.