In the genus Mustelus, to which our “ smooth dog-fish ”
belongs, the teeth deviate from the form typical of the sharks and
approach those of the rays, being obtuse and rounded, with fine transverse
ridges, without a piercing cusp or cutting edge, but adapted for
bruising and crushing: they are numerous, small, nearly equal,
arranged like a pavement in the quincuncial order.
The maxillary teeth of the saw-fish CPristisJ resemble those of the
smooth dog-fish, but as the more formidable part of the dental system
of this genus offers some peculiar characters, it will be subsequently
described.
In the Cestracion both prehensile and crushing teeth are associated
together in the same jaws, but as we here meet with a modification
of the microscopic texture of the teeth different from that
of the teeth in the true sharks, a separate section will be devoted to
their description, and to that of the teeth of numerous cognate fossil
species of plagiostomes.
In all the genera of sharks, the body of the tooth is principally
occupied with the two kinds of canals which I have termed “ medullary”
and “ calcigerous.”(l) The latter are, however, essentially
minute branches or continuations of the former, and although, in
the newly formed tooth distinguishable by the nature of their contents,
yet this difference is gradually obliterated by the progressive deposition
of calcareous matter by concentric layers in the medullary
canals.
In plate 6, a view of the structure of the tooth of an extinct species
of shark (Lamna elegans, Ag.) is given as seen in a thin longitudinal
section under a compound lens of one inch focus. With this power the
medullary canals alone are visible, the minute calcigerous tubes giving
rise to the cloudy appearance in their interspaces. The medullary
canals are continued from the short and small pulp-cavity at the base
of the tooth. The principal branches run parallel with the axis of the
tooth, but quickly give off raxnuli, which are directed transversely,
and again ramify at right angles or nearly so, and anastomose, so as
to form a beautiful reticulate arrangement of tubes, very similar to a
network of capillary vessels, throughout the whole substance of the
llj Report of the British Association, 1838, vol. vii, p. 135.
1 tooth : they ultimately terminate in flattened sinuses, which anasto-
Imose and form the boundary line between the central osseous and
I the dense exterior enamel-like substance of the tooth. The whole of
fthe superficial part of the tooth is occupied by minute calcigerous
■ tubes, (1) which proceed in a wavy course, generally at right angles to
■ the external surface ; they ramify at very acute angles, and their
Iterminal branches anastomose, and most of them end in a
Iseries of calcigerous cells, situated beneath the outer stratum of
■ enamel. In this stratum, however, there are evident traces of a
Iseries of much finer tubes, continued from the preceding layer of
fcells. The medullary canals in the body of the tooth are surrounded
;jby concentric layers, traversed by the calcigerous tubes which are
ileverywhere given off at right angles from the larger canals ; and
have a more irregular wavy course than the superficial calcigerous
(tubes. They form, by their numerous anastomoses an inextricable
|reticulation,(2) and their terminal ramuli dilate into or communicate
pith calcigerous cells.
The medullary canals of the teeth are occupied in the recent
dog-fish by a sanguineous medulla, closely resembling that which
fills the medullary cells of the coarse bone of which the base of the
tooth is composed, and with which cells the anastomosing reticulate
canals of the crown of the tooth are directly continuous. In the old
exterior teeth a great proportion of the medullary canals are consolidated
by concentric layers of earthy deposit.
In the fully formed flat teeth of the foetal Carcharodon, which
were sufficiently transparent to render their texture perfectly discernible
by transmitted light, this was seen to be essentially the same as
In the Lamna ; but the disposition of the medullary canals was more
regular : the median branches were continued as in Lamna to the
apex of the tooth parallel with the axis, the lateral ones gradually
inclined to the sides of the tooth, their direction being more transverse
as they approached the base ; and here inclining downwards to
■ he hilobed osseous root. The branches of the medullary canals are
(1) A highly magnified view of the calcigerous tubes continued from the peripheral medul-
lary tube b is given in PI. 7*
I (2) See the view of these tubes between the medullary canals a and b, PI. 7.
D