disposition is associated therewith. The testaceous and crustaceous
invertebrate animals constitute, most, probably, the principal food of
the Cestracion.
The teeth are attached to the maxillary surface by a slightly
contracted and truncated basis : the part which may be termed the
crown of the tooth is covered with a layer of dense white substance,
analogous to enamel, the surface of which is impressed by numerous
minute pits. Below the crown, the surface of the tooth is still
more irregular, and the basis is composed of coarse fibres with intervening
fissures and foramina. The small anterior teeth, which
resemble those of certain sharks, may be distinguished by their rugous
base, which is also broad and flat.
When the dense outer layer is removed from the crown of the
newly formed teeth, the orifices of the medullary canals or tubes perforating
the whole body of the tooth are brought into view.
The texture of this tooth, in fact, like that of the Myliobates, is
precisely such as would—suggest the idea of the tubular texture
assigned by Cuvier to the compound teeth of fishes. These tubes
of canals, which are visible to the naked eye, are more or less
occupied in the recent fish by a vascular and organized medulla or
pulp. They are continued directly from the irregular and reticularly
disposed cells and canals of the semi-ossified cartilaginous crust of
the jaw. In the large crushing teeth, the greater number of. the
medullary canals proceed in a pretty regular and slightly wavy course
towards the grinding surface, while the outer ones incline towards the
lateral surfaces ; but they soon begin to divide, and the divisions
continue to ramify dichotomously; the branches anastomose together,
especially near the surface, and form loops of which the convexity
is always directed but flattened, towards the unattached surface of
the tooth. The medullary canals are, in general, slightly dilated
before they dichotomize, and the branches maintain throughout nearly
the same size as the trunk.
Plate 12 shows the general appearance of the medullary canals
in a vertical section, including part of the lateral surface of the tooth,
as seen by transmitted light through a compound lens of half an inch
focus. The dark colour of the wide medullary canals is due to the
opacity occasioned by the earthy matter deposited in them from their
peripheral extremities to within a short distance of the base of the
tooth. The fine calcigerous tubes, from the same cause, appear as
dark lines in the interspaces of the medullary canals, and as they pass
into the enamel-like superficial layer.
In Plate 13 is exhibited a portion of two of the terminal medullary
loops, with the calcigerous tubes continued from them as seen
under a magnifying power of 600 diameters. The interspaces of the
medullary canals are traversed throughout the substance of the tooth
by calcigerous tubes, which have a general direction vertical to the
medullary canal from which they proceed.
The calcigerous tubes present, at their origin, a diameter, generally,
of sisWth of an inch. They are more ramified, and have a more
undulatory course than the medullary canals, especially those which
occupy the interspaces of these canals, where they form a moss-like
network, in the meshes of which are minute cells, with which the finest
branches of the calcigerous tubes are in communication. The calcigerous
tubes of the grinding surface of the tooth have a general
direction, vertical to that surface, and the enamel-like coating is
formed principally by the finest terminal branches of these tubes,
imbedded .in a transparent and apparently structureless matrix.
Under a power of -*th inch focus, longitudinal series of irregular and
partially confluent rhomboidal cells are discernible in many parts of
this external layer.
The jaws of the Cestracion, like those of the other sharks,
exhibit the teeth in various stages of formation. At the innermost
extremity of some of the rows, may be seen a small, flat, punctate,
milk-whitej calcareous plate, of a friable texture, resulting from the
recent deposition of the earthy particles in the microscopic cells and
tubes of the superficies of the formative matrix ; the only part which
as yet is developed from the common mucous and ligamentous basis
of the teeth. The rest of the matrix is progressively added until
the tooth acquires its full size; the deposition of the calcareous salts
proceeding from the crown to the base simultaneously along the whole
breadth of the tooth.
The process of dentification is here most clearly seen to be one