sent such modifications of their common and characteristic type of
structure as fits them for very different habits of life and the acquisition
of different kinds of food. The active, and predatory sharks
are here associated with the sluggish omnivorous rays, and the
dental system presents every grade of modification from the laniary
to the molary type ; the Lamna with its teeth exclusively adapted
for holding, piercing, and lacerating, and the Myliobates with its
maxillary mosaic pavement of flattened molars forming the two
extremes of the series.
13. . The sharks, or Squaloid plagiostomes, with few exceptions,
have teeth of a conical, sharp-pointed, more or less compressed form ;
sometimes with trenchant or serrate edges and accessary basal denticles
; they are arranged along the margin and posterior surface of
the jaws in close-set vertical rows, of from three to thirteen teeth in
each row, according to the species. The teeth of the contiguous rows
in certain genera, as Selache, and Lamna, are parallel with each other,
but in Galeus, and Carcharias, they are placed alternately, so that the
base of one tooth advances laterally into the interspace of two teeth
of the contiguous row, and reciprocally; hut the laterally contiguous
teeth are never articulated with each other as in certain rays. In
the Scymnus, the median row of teeth crosses the symphysis of the jaw,
and their base overlaps the adjoining margins of the contiguous teeth;
the lateral teeth have an imbricated arrangement.
In general the anterior or external tooth only of each row is
erect, the rest being recumbent; the contrast, in this respect, is most
marked in the lower maxillary lancet-shaped teeth of the Scymnus,
(PI. 4, fig. 3). In Lamna, however, the second and third teeth are
commonly seen in positions intermediate between those of the erect
anterior and the recumbent posterior teeth, (PI. 5, fig. 1,) and in the
rays where the teeth are much more numerous in each row than in
the sharks, they exhibit every gradation between the recumbent,
reflected, erect, and porrect positions. It is scarcely necessary to
repeat, that although the teeth of the sharks possess greater individual
mobility than those of the rays, the recumbent ones cannot, as has
been supposed, be voluntarily erected; these teeth are still in
progress of development, and several of them are covered by a reflection
of the mucous membrane of the mouth, which would be
lacerated by such a movement; it is by a gradual change of position
in the fibrous membrane to which their base is attached, that the
altered direction of the consolidated teeth is effected.
The teeth present the smallest relative size among the sharks,
in the sub-genus Rhinodon of Dr. Smith, where they may be compared
with the teeth en brosse, of certain osseous fishes ; here they
are of a simple conical, slightly recurved form; there are twelve or
thirteen teeth in each vertical row, and about two hundred and fifty
of such rows in each jaw.
In the sub-genus Selache, to which the great basking shark,
I (Squalus maximus, Home), belongs, the teeth, though small, are relatively
larger than in Rhinodon. They are conical, recurved, and with a
somewhat obtuse apex. In a specimen about thirty-six feet in length,
the teeth, which are alike in both jaws, measure not quite half an
inch in length, and between two and three lines across their rounded
base. The sharks with teeth of larger size and more formidable
aspect present many modifications of shape, by which, with other
characters, the genera and sub-genera of squaloids are distinguishable.
The principal varieties of form are illustrated in plates 3 and 4. Varieties
of form, however, it should be remembered, are not only indicative of
generic distinctions, but sometimes of a difference of age of the same
individual. In the common spotted dog-fish, for example, the one
or two lateral denticles at the base of the principal cusp described
as characteristic of the teeth of the genus Scyllium, frequently disappear
in the old fishes. In the young of the blue-shark (Carcharias
glaucus) the teeth have smooth trenchant edges, but in the old ones,
the margins are dentated. In many genera, again, the teeth of the
upper differ in form from those of the lower jaw ; this is most remarkably
the case in the genus Scymnus: they also frequently differ
in shape as well as in size in different parts of the same jaw. But
while a knowledge of these facts should impress the observer with
due caution in giving an opinion on the specific or generic relations
of an extinct shark from the examination of a single tooth, the peculiarities
of form characteristic of different genera of existing squaloids
are sufficiently constant and well marked to render dental