surface of the tooth. The relative thickness of this layer of fine-
tuhed dentine is shown in the figure ; the diameter of the calcigerous
tubes at the middle of the layer does not exceed j^ th of an inch.
The traces of the corresponding tubes in the clear interspaces of the
ramifications of the medullary canals were almost obliterated in the
fossil examined by me, but the analogy of the recent Scomheroid teeth,
and of some allied fossils, especially that about to be described, hardly
permits a doubt as to their existence.
Saurocephalus. — There are few instances in which the value
of the characters derived from the microscopic structure of teeth
has been more strikingly displayed than in regard to the Saurocephalus
and Saurodon, under which names two interesting fossils of
distinct species, of the same or very nearly allied genera, have been
described as extinct members of the Saurian Order of Reptiles, by
Dr. Richard Harlan(l) and Dr. Isaac Hays. (2)
These fossils consisted of portions of jaws with teeth of a simple
conical subcompressed form, arranged in a single row, fixed in distinct
and deep alveoli, each with a broad and simple fang, generally
excavated by the pressure of a new tooth developed near its base.
Prof. Agassiz was led from the external characters of the jaws and
teeth in question to believe that they might belong to the Scombe-
roid family in the class of Fishes, and an inspection of Plate 55,
which gives a view of a small portion of a transparent longitudinal
section of one of the teeth of the Saurocephalus, will demonstrate the
accuracy of the judgment of that acute Palaeontologist.
I am indebted to Dr. Richard Harlan for the opportunity of
making the requisite sections of the tooth of the genus in question.
The plan of structure closely corresponds with that already described
in Sphyrcena and Sphyrcenodus. The larger medullary tubes (a a,
PI. 55), maintain a nearly parallel longitudinal course, throughout
the body of the tooth, exhibiting here and there a dichotomous subdivision,
and gradually decreasing in diameter as they approach the
summit of the tooth. The lateral or transverse branches are, upon
(1) Journal of the Academy of Natural Sciences of Philadelphia, vol. 3, p. 331.
(2) Transactions of the American Philosophical Society, vol. 3, part 2, p. 471. In the
Saurian system of Herm. v. Meyer these genera are placed between Phytosaurus and
Teleosaurus, see his Pakeologica, 8vo. 1832, p.p. 114, 222.
the whole, relatively larger than in the Sphyrænodus, and are more
flexuous in their course ; and the spaces intercepted by their anastomoses
are less angular. These spaces (6) exhibit every where
plexiform groups of flexuous calcigerous tubes proceeding from the
medullary canals. The clear and dense investment of the tooth
presents the same structure as the peripheral dentine in other Scom-
beroid fishes, being traversed by suh-parailel and acutely-branched
calcigerous tubes, passing from the nearest medullary tubes, at right
angles to the surface of the tooth, and diminishing to extreme tenuity
in the clear outer portion of the investing layer, c. No Saurian tooth
recent or extinct has presented the type of structure here described :
the distribution of branched and reticularly anastomosing medullary
or pulp-canals through the whole body of the tooth is a peculiarly
Ichthyic condition of the dental structure ; and the modifications of
thiê condition presented by the tooth of the Saurocephalus, are most
closely allied to those which characterize the teeth of the Scomheroid
Fishes.
In Plate 55 an entire tooth of the Saurocephalus lanciformis and
a portion of jaw, containing two others imbedded in their sockets,
are represented of the natural size, by the side of the magnified section.
The small foramen opposite the fang of each tooth is described
by Dr. Hay, as being for the transmission of nerves and blood-vessels
to the teeth. I have little doubt, from the analogy of the Sphyræna,
that these foramina lead to the cavities containing the germs of the
successional teeth. They are placed as in the Sphyræna, on the inner
side of the alveolar process.
LUCIOIDS.
53. The fishes of the family typified by the voracious Pike, and
hence termed ‘ Lucioids,’ have a more complicated dentition than those
of the preceding family ; but the teeth are characterized by a similar
reticulo-medullary tubular structure, distinguished chiefly by the
more regular size and form of the meshes or interspaces of the anas-
tomostic net-work.
The Scomheresox is a genus which connects the Scomheroid with
the Lucioid family of fishes. The jaws of a species of this genus from
k 2