size of these teeth distinguish the primary subdivisions of the same
Order. A few other groups of Fishes are well defined by dental characters,
as the Pycnodonts, Gymnodonts, Goniodonts, and Chsetodonts.
But in most of the natural Orders, and in many of the subordinate
groups of the Piscine class, the dental system is subject to very great
diversity in regard to the form, number, and position of the teeth;
and in some natural families there is also a want of constancy in the
structure of the teeth. There are extremely few genera of Fishes that
can be characterized by a definite numerical dental formula, like
most of the Mammalian genera. Indeed, in the first introduction
of true teeth into the animal series, regarded in the ascending order,
they manifest, like the mouths of the Polypi, the stomachs of the
Polygastria, and the generative organs of the Taniee, the principle
of vegetative or irrelative repetition; and, in many Fishes, are
too numerous to be counted. The limits within which the teeth
are applicable as means of classification in Fishes will be readily,
and I trust, accurately appreciated, by the descriptions in the first
part of this Work. Traced from species to species they are of
great importance in the determination of the fossils of this class.
With regard to microscopic structure, the second and third
of the modifications defined in Chapter I, Section 8, are peculiar
to and characteristic of the Piscine Class ; the first modification is,
with the exception of one Mammalian genus, Orycteropus, peculiar
to Fishes; unvascular or fine-tubed dentine forms the crown of
the teeth in a few Fishes, but is more common in those of the
higher Classes.
In the Class Reptilia the teeth serve to characterize smaller and
more definite groups than in Pisces, as, for example, the venomous and
non-venomous Ophidians; and the acrodont, pleurodont, and thecodont
Saurians. Certain Genera, and even Species may likewise be known
by peculiar forms of teeth; but a definite dental formula can rarely
be assigned as a generic character of a Reptile. There is no decided
modification of dental structure peculiar to any of the class
of Reptiles; the poison-fang is rather a modification of form.
The labyrinthic structure reaches its maximum of complexity in
the great extinct Sauroid Batrachians of the Keuper Sandstones,
but “ it also exists at the base of the tooth in a few Fishes,”
(Part II, p. 201), and specific instances of it in that class (Lepi-
dosteus, and a few other Sauroids) have, since Part II. was published,
received illustrations in the works of Prof. Agassiz(l) and Dr.
Wyman. (2) The only constant and general character of the teeth of
the cold-blooded classes of Yertebrata is derivable from the brief
period of their existence in the individual, so that the few teeth which
develope roots have these always simple and undivided, usually
hollow, and with the germ of a successor in or near them.
With the exception of the composite dental masses of the
Chimseroids, and the anomalous rostral teeth in Pristis, no existing
species of Fish or Reptile could be said to have permanent teeth;
and no extinct species of either class has yet been found with teeth
having divided roots implanted in sockets, or manifesting evidence
of perpetual growth by a persistent pulp, excepting the singular
extinct Saurians of South Africa, with two long canine tusks in the
upper jaw, which must have grown and been maintained throughout
life, of due size and strength, like the tusks of the Boar and
Walrus.(3) With the exception of these two anomalous teeth,
the jaws of the Dicynodonts were edentulous.
In the Mammalian Class the value of the dental organs, as
(1) Poissons Fossiles, Notice surles Sauroides, Janvier, 1843.
(2) Trans. Boston Society of Natural History, August, 1843.
(3) Memoir on the Dicynodon, Geological Transactions, 2nd series, vol. vii.