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380 DENTAL ANATOMY. ;

It slioulcl be stated here that some of the tailless species (toads) are
quite edentulous, M'hile in others (the frogs) teeth are absent in the lower
jaw. All the existing tailed batrachians, however, are provided with
teeth which present practically the same pattern and disposition which
obtains throughout tlie entire sub-class.
An excellent and easily-obtained example of this latter subdivision is
found in the Alleghany 3Ienopoina, popularly known as the " hell-
bender." In this animal the skull is remarkable for its flatness and
breadth, as well as the almost perfect semicircular outline ^liich the
dentigerous surface of the jaws presents. The mandible, as in the fishes,
is composed of angular, articular, and dentary pieces, and is suspended
to the cranium by means of two bones known as the squamosals through
the intervention of the quadrates.
The palato-quadrate or palato-pterygoid arch is not so well defined,
although the principal elements are present. The vomers are two in
number, and occupy their usual position behind the maxillaries, sharing
in the formation of the bony roof of the mouth. The maxillaries and
premaxillaries also have the same position as in the fishes, but are less
mobile, on account of sutural connections with the surrounding bones.
The l)iting surface of each jaw is produced into a sharp ridge by rea-
son of the existence of a well-marked ledge extending the full length of
its internal face. This ledge is converted into a groove in the recent
state by a fold or flap of the gum, which forms its internal wall, and is
in all probability homologous \\\\\\ a similar structure (the thecal fold)
found in the sharks. At the Ijottom of this groove the tooth-germs of
the successive sets of teeth are developed. It will be seen, therefore,
that the general arrangement is not diiferent from that of the sharks
but this important difference is to be observed : in the sharks the bases
of the teeth are at first directed upward, and it is only when they are
about ready to take position on the working surface of the jaw that they
assume the erect attitude ; this, as we have already seen, ife clue to the
movement of the entire gum outward. This manner of replacement
gives Mcight to the conclusion that the teeth in the sharks are invagi-
nated dermal spines, the position of which we would expect to find
reversed upon the inside of the jaws.
In the batrachians. on the contrary, the teeth are said to have an erect
position from the earliest stages of development, and it is less easy to
see how they represent dermal spines or how the position came to be
reversed. Believing, however, that all the maxillary and mandibular
teeth were originally of tegumcntary origin, as is clearly demonstrable
in the sharks, it is more than probable that the arrest of the outward
movement of the gum in the batrachian by the appearance of ossifi-
cations around Meckel's cartilage to form the dentary bones is responsi-
ble for this change. It is quite possible that the tooth-germs of the
batrachian do at first have the same position as those of sharks—that is
to say, with the points directed downward—and that the formative
energy of the tissues beneath causes them to become erect at a compara-
tively early period.
We have already stated, in connection with the account of the dental
organs of the elasmobl-anchs, that tooth-succession is primarily due to