is not unusual to find the capsules of contiguous tooth-germs becoming
adherent together as their ossification proceeds. After a brief maceration
the soft gum may be stripped from the shallow alveolar depression
and the younger tooth-germs in different stages of growth are
brought away with it.
The mode of development of the teeth of serpents does not differ
essentially from that of the teeth of the Batrachian above described,
except in the relation of the papillae of the successional poison-fangs
to the branch of the poison duct that traverses the cavity of the loose
mucous gum in which they are developed.
The situation in which the successional teeth are developed in
the Yaranus is shown in PL 63 a , fig. 9 : their relative position to,
and the mode in which they affect, the adherent teeth, by pressing
upon the inner side of their bases, is shown in the upper and lower
jaws of the Cyclodus (Plate 66, fig. 7 b) j the phenomena of dental
development in these and other lizards closely correspond with those
described in the frog. In the Acrodont lizards, and those in which
the teeth are anchylosed to the summits of bony processes, the successional
teeth are in like manner developed at the inner side of
the supporting processes, gradually penetrate them as their growth
proceeds, and finally undermine and displace the tooth and become
in their turn anchylosed to new bony eminences of the alveolar
tract. The jaws of the gigantic Mosasaur exhibit on a large scale
different stages of this mode of shedding and replacement, whieh
is so general in the class of reptiles.
In the Ichthyosaurus, in which, by the development of an internal
as well as an external alveolar plate, the teeth are lodged in a deep
continuous groove, the successional germs were also developed in this
extinct reptile at the inner side of their predecessors, and, from the
solidification of the implanted base of the fully formed tooth, occasioned
an extensive absorption of its inner side, before it finally
yielded to the lateral pressure.
In the Crocodile, the tooth-germ is developed from the vascular
membrane covering the base of the internal wall of the socket; it is soon
invested by a capsule, and by its pressure causes the formation of a
shallow recess, or secondary alveolus, in the contiguous bone (PI. 75,
fig. 4). In this alveolus, however, it never becomes inclosed like the
successiohal teeth in most mammalia; for, exerting equal pressure
against the fang of the contiguous tooth, which, from being incompletely
formed, has a wide pulp-cavity with very thin walls, the nascent tooth
soon penetrates that cavity, and quits the recess in the alveolar plate
in which it was originally situated. Thus the stage of development
corresponding with the ‘ eruption’ of the tooth in the mammalia
is immediately followed by the ‘ inclusion’ of the new
tooth in the pulp-cavity of its predecessor. Further details of the
development and succession of the teeth of the Crocodiles will be
given in the chapter appropriated to the description of the dental
system in that family; but I may here observe that the rapid succession
of tooth-germs, which stamps the impress of decay upon their
predecessors often before the growth of these is completed, though
common to many reptiles, is most strikingly manifested in the
Crocodiles, in which three and sometimes four generations of teeth,
sheathed one within the other, are contained in the same socket.
C H A P T E R II.
TEETH OF BATRACHIANS.
75. The variations which the dental system presents in the Batrachian
order of Reptiles are more conspicuous in the number, situation,
and structure of the teeth, than in their form or mode of attachment.
Certain Batrachians are edentulous, as the genus Hylaplesia,
among the tree-frogs, and the Bufonidas or family of Toads, some of
the species of Bombinator excepted.
The teeth when present are generally numerous, simple, of small
and equal size, and close-set, either in a single row or aggregated
like the teeth of a rasp.
It is not without interest to observe that a characteristic condition
of the dental system in fishes, viz : the absence of teeth on the
superior maxillary bone, is continued in those genera of perenni