Page 368 - My FlipBook

378 DENTAL ANATOMY.

larly in size toward the back part of the jaw, and have notched cntting
extremities.
The teeth of the posterior row are applied closely to those of the
front row, and are completely concealed when the mouth is closed.
They have a broader, more incisiform pattern. The teeth of the lower
series are like the corres])onding ones of the upper front row, and are
lodged in the dentary bone. They are attached by slight ankyloses to
the respective bones upon which they are supported by having their
bases placed in a shallow alveolar depression, in the middle of which
a conical process of bone rises up and is received into the hollow basal
portion of the tooth. The successors of those in use are developed deep
clown in the substance of the jaws, in bony crypts which communicate
with the exterior by means of foramina in the side of the jaws in the
vicinity of the bases of those in use. All the teeth in this species are
said by Owen to be covered with enamel. There are likewise small
conical hooked teeth developed upon the pharyngeals.
While the teeth of Balistes are more nearly affiliated with the normal
teleost condition, those of Dlodon, on the other hand, show a much
wider departure. When the mouth is closed the biting surface of the
jaws seems to be invested with a continuous covering of tooth-sub-
stance ; upon close inspection this is found to consist of a number of
dentine plates closely incorporated with the bone of the jaws and more
or less fused together at the base. Each one, however, develops sepa-
rately, and takes its place when its predecessor has disappeared through
Avear. Just inside the margin of each jaw, in the middle line, is to be
seen a broad rounded mass consisting of transverse plates of dentine
intimately blended and ankylosed to the jaw bones. A faint median
longitudinal suture divides each into two parts ; when this becomes
more distinct and extends to the edge of the jaw, as it does in some
species, it constitutes the mark of generic distinction of the genus Tetro-
don. The plates composing this mass, Mhich is peculiar to these fishes,
are developed in the same manner as the teeth, and are strictly homol-
ogous with them.
Many other examples quite as peculiar as the last one could be cited
among the dental organs of fishes, but more time and space would be
required than is aiforded the present paper.
The mode of develo})ment is essentially the same as that described
for the mammal, with the exception that the dental sacculus is generally
simple ; the dentine papilla arises from the corium, and the oral epithe-
lium dips down to form a cap-like investment, in both of M'hich calci-
fication takes j)lace in the manner already described. AVith regard to
the succession and attachment of the teeth in this group, as well as in
the Batrachia and Rei)tilia, some preliminary points in the development
of the jaw bones must first be noticed.
In tlie early stages of embryonic development each jaw is primarily
made up of two cartilaginous bars which meet in the median line in
front. In the upper jaw these bars are known as the palato-ptervgoid
bars, and in the lower jaw as Meckel's cartilage. In the elasmobranchs
these bars persist, and the teeth are supported Iw them. As a conse-
quence of this condition, as we have already seen, succession in them