merits drawn from the rest of its organizational) that the Lepidosiren
is in every essential point a member of the class of fishes.
SAURICHTHYS.
65. I terminate the description of the dental system of fishes with
an account of the microscopic structure of the teeth of two extinct
species of different genera, indicated as yet only by these detached
organs, and respecting which, doubts have been entertained as to
whether they be actually referable to the class of fishes or of reptiles.
To these doubts, however, in regard to the first genus, Sau-
richthys, (2) the microscopic test satisfactorily puts an end ; but with
respect to the other genus, Dendrodus, the modification of the dental
tissue approaches so closely to a new and singular condition which
will be shown to characterize the structure of the teeth of an extinct
family of Batrachian reptiles, that I hesitate in absolutely pronouncing
it to be that of a fish.
The teeth of the Saurichthys characterize the Bristol bone-bed
and the German Muschelkalk ; they are conical, rather slender, with
a subelliptical transverse section, slightly bent, with the apex not
very acute : about one fifth of the tooth next the apex is smooth and
polished, the remainder of the outer surface is traversed by fine close-
set longitudinal ridges : thus, to all outward appearance the tooth
very closely resembles the form most common among Saurians.
The specimen here described, from the Bristol bone-bed, is seven
lines in length, and two broad at the base. I have investigated its
structure by a transverse section through its base, and by a longitudinal
section through the middle of the remaining part.
This tooth is traversed by a slender, straight, longitudinal,
conical, central pulp-cavity, occupied by a coarse cellular bone, and
from which radiates a system of close-set, slightly and minutely
undulating calcigerous tubes, covered by a thick layer of the
enamel-like dense tissue, which coats the teeth in the Lepidotus,
(1) Linnsean Transactions, vol. 18, 1839. p. 350.
(2) The term Saurichthys (aavpoQ a lizard, a fish) was applied by M. Agassiz to the
present genus, to express its transitional characters, but it is nevertheless regarded by that
distinguished Palaeontologist, as being essentially a member of the class of fishes.
Sparus, and many other fishes. The central linear pulp-cavity
extends, as also in the conical teeth of many fishes, to the extreme
apex of the dentine : the cells of the ossified pulp were occupied by
an opaque substance in the tooth examined: at its periphery and
through the whole of the base of the tooth were scattered numerous
coarse cells. The true dentine presented a mahogany brown colour :
the main calcigerous tubes of this substance radiate through it at
right angles to the periphery of the tooth, and with a slight curvature,
the concavity of which is turned towards the apex of the tooth. The
tubes give off many small lateral branches, which are curved, with
their concavity directed towards the pulp-cavity, or to the base of the
stem: they diminish in size as they approach the external enamellike
substance, pass across the well-defined boundary which separates
this substance from the dentine, and then are immediately
resolved into fasciculi of the extremely fine and slightly diverging
fibres or minute tubes which form the only appreciable structure in
this enamel-like substance. These fine lines have a general direction
at right angles to the periphery of the tooth, like that of the calcigerous
tubes of the dentine, from which they are continued; but many
of them are bent in different directions, so as to cross each other, in
the manner described and figured in the corresponding clear external
enamel-like substance of the tooth of the Lepidotus ; they are,
however, somewhat larger and diverge in straighter lines ; they finally
terminate in fine calcigerous cells at the periphery of the tooth.
The thickness and structure of the dense external enamel-like
layer, the length and form of the pulp-cavity, and the texture of the
ossified remains of the pulp with which it was filled, are decisive
against the reptilian character of the Saurichthys. Its affinities to
that class are manifested only in the outward form of the tooth, and
the dense, minutely-tuhed tissue of the dentine, a structure, however,
which is common to the teeth of all the Sauroid fishes.
DENDRODUS.
66. Dendrodus biporcatus.—Theteethindicativeof the genus, for which I
haveproposedthe nameofZ)e?idrodMs(l), occur sparingly and detached in
(1) SkvSpov a tree, odae a tooth; in reference to its internal dendritic structure.