towards the circumference of the tooth, and consequently at the two
extremities of the section, where only the structure above described is
visible. Several small twigs pass beyond this plexus into the clear
enamel-like outer layer of the tooth, in some parts of which traces were
perceptible of a plexus of still more minute tubes, or strice, which gradually
diminished until they escaped the highest magnifying power
employed in this examination. (PI. 33). The enamel-like layer is
clearly a continuation of, and part of the same substance as the rest of
the tooth $ its dense and clear texture indicates the extreme subdivision
and abundance of the earthy salts : this stratum is of considerable
thickness in the Sphcerodus crassus of Agassiz.
In the genus Gyrodus the surface of the teeth is furrowed sometimes
irregularly, sometimes regularly and deeply as in Gyr. rugulosus.
(PL 34, figs. 6 and 7). The teeth are present in the intermaxillary, pre-
mandibular, palatine and vomerine bones.
In the Gyrodus umbilicatus each premandibular bone supports four
rows of teeth (PI. 34, figs. 4 and 5); those of the external and third rows
are of a transversely oval form, and are larger than those of the second
and internal rows, which have a circular contour. The intermaxillary
and premandibular teeth are fewer in number, and present an obtuse
conical form in the Gyr. circularis. The vomer in this species is
covered with five longitudinal rows of teeth; those of the middle one
being the largest as in the Pycnodus.
In the tooth of a Gyrodus cretaceus, I find that the tendency to the
structure of the dense ivory of the teeth of the higher Vertebrata, which
is obvious in the teeth of Splusrodus and Lepidotus, is carried on to a
close correspondence. The base of the tooth is excavated by a large
and simple pulp-cavity, presenting a quadrate figure in a vertical section
of the tooth ; this cavity is immediately continuous with the large
cells and reticulate canals of the bony base. The body of the tooth
consists of close-set minute calcigerous tubes, having a diameter of
yLAh of a line at their origin, radiating in a direct line, but with a
minute and regularly undulating course, and a gradually diminishing
diameter to the superficies : the lateral tubes pass horizontally, and
those continued from the summit of the pulp-cavity vertically, to the
grinding surface. They give off very regular, but extremely minute
branches, which are lost in the clear and dense enamel-like superficial
laver of the tooth.
In the genus Microdon, the teeth, though presenting all the
general characters of the Pycnodontal structure, are reduced to their
smallest dimensions. They present a flattened angular form, and are
arranged in many rows on the intermaxillary, premandibular and
vomerine bones. The whole substance of the tooth is composed of
calcigerous tubes, which are straighter, more parallel, and relatively
finer than in Gyrodus, whence there results a texture of still greater
density, and one that approaches very closely to that of the molar teeth
of the Gilt-head (Chrysophrys). The arrangement of the component
tubes of the tooth of Microdon is shown in PI. 43, fig. 1.
The pavement of thick, round, convex or flattened teeth of the
genera of Pycnodonts, above described, was adapted, like the corresponding
teeth of existing fishes, to break and crush small testaceous
and crustaceous animals. These teeth, under the name of Bufonites,
occur most abundantly in the oolite formation.
The teeth of the extinct fishes of the genus Placodus present
reverse proportions to those of the Microdon, and here attain their
maximum of development in the Pycnodont family. Plate 30, fig. 2
exhibits the alveoli of the prehensile intermaxillary teeth, and the
I molar teeth of the same bone in situ of the Placodus Andriani, Ag.
The prehensile anterior teeth were arranged in two transverse rows
of six in each row. The anterior were the largest, and presented a singularly elongated cylindrical form, with a conical slightly recurved
obtuse crown (fig. 3). In Placodus gigas, the crown of the incisor is
j more expanded and more abruptly bent. These teeth, like the divergent
anterior teeth of the Wolf-fish, must have served to grapple with
large Testacea or Crustacea, the crushing and comminution of which
were then completed by the posterior dental plates. These are
arranged in four rows, of which the two external ones include each
i four smaller subcircular teeth ; the two internal rows consist each of
I three large tetragonal dental plates with the angles rounded off, and
the upper surface flattened and smooth.
In the lower jaw it would seem that three similar dental plates in