surface near the base, to a continuous alveolar groove defended by an
external wall of bone. In the common frog (Rana temporaria), there
are about forty of these teeth on each side of the upper jaw, of which
eight are supported by the intermaxillary bones. In the Rana esculenta,
(PI. 62, fig. 8), there are the same number of intermaxillary, but a
greater number of maxillary teeth. In the great bull-frog, (Rana
pipiens) there are ten or twelve intermaxillary teeth, followed by
between sixty and seventy maxillary teeth (PI. 62, fig. 10). The
largest teeth are placed in the anterior third of the series, whence
they progressively diminish in size towards the two extremities. The
vomerine bones each support a short transverse row of four or five
small teeth. The base of the teeth, besides being confluent with
the bone, are, from their close contiguity, frequently anchylosed to
one another: they are sometimes thus conjoined to near the apex.
Where the larger teeth"are situated, the alternate ones are commonly
found to be displaced. The germs of the successional teeth are
developed in a groove of the mucous membrane of the mouth which
covers the inner side of the basis of the teeth ; as they increase in size
they press upon and cause absorption of the contiguous surface of the
base of the old teeth, and thus finally displace them and become in
their turn anchylosed to the hone, then undermined and shed. As a
general rule the toads (.Bufonidce) are edentulous, but among the
Bombinatores, or those species which are termed ‘ earless,’ from
having the tympanum concealed under the skin, the subgenus Hyla-
dactylus has teeth upon the vomer and Sclerophrys has teeth on
both the intermaxillary and maxillary bones. (1 )
In microscopic structure, the teeth of the existing Batrachians I
like those of most Saurians, correspond with the simple mammalian
teeth.
If a longitudinal section of the tooth of a frog, including a portion
of the alveolar plate to which it is anchylosed, be examined by transmitted
light under a half-inch objective, a narrow transverse line will
be seen inflected inwards from both the external and internal side of
its base, defining the limits of the tooth and hone. Below this line
the bone is characterized by numerous large, oblong, radiated cells,
(1) M. Tschudi, 1. c. p. 3.
having their long axis vertical, or in the direction of the axis of the
tooth: these cells do not extend into the substance of the tooth
itself. The dentine is composed of fine calcigerous tubes and the
intermediate clear substance, the minute cells of which are unusually
conspicuous and abundant. In a completely formed tooth no trace of
a pulp-cavity is discernible in the prominent crown. The calcigerous
tubes are continued into the body of the tooth at once from
the subjacent hone, in the interspace of the transverse fissure first
mentioned. The tubes nearest the external and the internal periphery of
the base of the tooth incline with a gentle curve towards those surfaces;
but the greater number proceed vertically, and nearly parallel with
each other, to the convex line anterior to the inflected apex of the
tooth, where their extremities are slightly bent outwards, except in a
few of the tubes nearest the apex which follow its inward curvature.
The secondary undulations and branches of the calcigerous tubes are
very elegant and conspicuous : the terminal branches of these tubes
end in a rich border of calcigerous cells. The dentine is not complicated
with medullary canals or inflected folds of the external
cement. The convex margin of the upper half of the tooth is coated
with a layer of enamel. The concave border and the basal half of
the opposite margin exhibit a thin layer of cement. The enamel,
viewed by transmitted light, presents the same dull brownish tint as
that of the saurian and mammalian teeth ; and has a minutely undulating
fibrous texture.
LABYRINTHODONTS.
87. The dental system in the extinct genus Ldbyrinthodon is more
formidable than that of any existing Batrachian : and principally
differs, as regards its more obvious characters, in the implantation
of the teeth in distinct sockets, and in the development of certain of
the anterior teeth of both jaws into large and formidable tusks.
A close-set series of subequal teeth extends along the alveolar
border of both upper and lower jaws, and along the anterior part of
the outer margin of each broad vomerine hone : two or three canineshaped
teeth, at least three times the size of the serial teeth, are placed
in the intermaxillary bones and at the anterior and external angle of
o 2