SHARKS.
given off nearly at right angles, but are relatively smaller and shorter I
than in Lamna. The calcigerous tubes in the body of the tooth form I
a fine but inextricable moss-like reticulation; those at the periphery I
are straighter, and run parallel to one another, and vertical to the I
outer surface. A stratum of fine calcigerous cells receives the termi-1
nations of these peripheral tubes, and intervenes between them and the I
dense enamel-like exterior covering of the tooth.
In the great fossil teeth of the Carcharias megalodon, the calcige-1
rous tubes, at the superficies of this tooth, are disposed in groups I
which, with an insufficient magnifying power, appear like single 1
coarse tubes; but with a higher power are seen to be composed of I
congeries of parallel tubes, apparently twisted together. The inter-1
spaces are nearly equal to the diameter of these fasciculi: they are!
occupied by more scattered tubes, and by short oblique or transverse I
anastomosing branches. At one part of a section of this tooth, which!
was sufficiently transparent to be examined with the highest powers,!
the peripheral coarse sinus or canal, which always runs parallel with!
the superficies, gave off an infinite number of minute tubes, whiclil
formed a plexus, (or plexiform stratum), and from the outer part of this ■
plexus, the tubes above described, passed, at right angles, to the sur-l
face. In the longitudinal section of this tooth, the twisted appearance!
above described of the peripheral calcigerous tubes was seen to be!
due to the number of side branches given off at an acute angle, and!
obliquely to the main tube. At the apex of the tooth, the marginal!
calcigerous tubes radiate, as in the Lamna, and suddenly diverge to!
proceed transversely to the sides. In the body of the tooth, the main!
canals are surrounded by concentric lamellae, traversed by radiating!
and anastomosing calcigerous tubes, which form a fine net-work ini
the interspaces of the medullary canals.
In the flat lancet-shaped teeth of the lower jaw of the Scymm«!
the texture of the tooth presents a closer correspondence with that ill
the higher organized animals, in consequence of the principal or me-l
dullary tubes being relatively smaller, more aggregated, straighter, and!
more parallel in their course, than in the sub-genera of sharks just!
described. These tubes advance in two fasciculi from the osseo®|
bifid base; the median tubes slightly converge and proceed straight!
to the apex of the tooth, the lateral ones also run parallel to each
other, and to the axis of the tooth, at the beginning of their course,
and then bend gently outwards to the margins of the tooth. The
secondary curvatures of the medullary tubes are pretty regular, and
of an angularly undulating character; the whole of the clear outer
enamel-like covering consists of parallel and extremely minute calci-
jgerous tubes with intermixed cells. The calcigerous tubes, in the
body of the tooth, are given off at an acute angle from the medullary
tubes through their whole course. I It has been already observed that the formation of the teeth of
the sharks, as of many other fishes, exemplifies, on a large scale, the
jearliest or papillary stage of dental development in the higher classes
of animals. It is not succeeded here by either a follicular or an
eruptive stage; the formative papillae are never inclosed, and con- Ijsequently never break forth. The pulp, when consolidated by the depo-
Isition of the calcareous salts in the pre-existing cells and tubes, is
■ gradually withdrawn from the protective sheath which the thecal fold
■ of mucous membrane afforded it during the early stages of its forma-
Stion. I have studied the development of the teeth of the squaloids,
fin the genera Galeus, Carcharias, and Scymnus. In the uterine fcetus,
■ one foot long, of the great white shark, (Carcharodon) the jaws seem at
Ifirst sight to be edentulous ; a fissure presents itself on the inner side
1 of the margin of each jaw running parallel with it, between the thin (■ smooth membrane covering the convex edge of the cartilage, and the
free margin of a fold of mucous membrane which lies parallel to, and
upon the inner side of the jaw. When this fold is drawn away from
■ the jaw, the minute teeth are exposed, arranged in the usual vertical
■ rows ; their points are all directed backwards and towards the base
■ of the jaw, and are seen to slip out of fossae, or sheaths in the
|membranous fold, as this is gradually reflected backwards to
Jits line of attachment near the base of the jaw. Here the an-
fterior lamina of the fold, which, from its office may be
■ termed “ thecal,” is continuous with the mucous membrane at the
B>ase of the rows of teeth; the posterior layer is reflected baek-
«wards to the fraenal line of attachment of the tongue. Close to the
»anterior line of reflection there is a row of simple conical papilke, in
d 2