smooth horny spines connected with the dorsal
fin. In the Cestracion Philippi alone, (PI. 1,
Fig. 18), we find a bony spine armed on its
concave side with tooth-like hooks, or prickles,
similar to those that occur in fossil Ichthyo-
dorulites: these hooks act as points of suspension
and attachment, whereby the dorsal fin is
connected with this bony spine, and its movements
regulated by the elevation or depression
of the spine, during the peculiar rotatory action
of the body of Sharks. This action of the
spine in raising and depressing the fin resembles
that of a moveable mast, raising and lowering
backwards the sail of a barge.
The common Dog-Fish, or Spine Shark,
(Spinax Acanthias, Cuv.), and the Centrina
Vulgaris, have a horny elevator spine on each
of their dorsal fins, but without teeth or hooks;
similar small toothless horny spines have been
found by Mr. Mantell in the chalk of Lewes.
These dorsal spines had probably a further use
as offensive and defensive weapons against voracious
fishes, or against larger and stronger individuals
of their own species.*
The variety we find of fossil spines, from the
Greywacke series to the Chalk inclusive, indi*
Colonel Smith saw a captain of a vessel in Jamaica who
received many severe cuts in the body from the spines of a Shark
in Montego Bay. (See Griffith’s Cuvier).
The Spines of Balistes and Silurus have not their base, like
that of the spines of Sharks, simply imbedded in the flesh, and
FOSSIL RAYS. 291
cates the number of extinct genera and species
of the family of Sharks, that occupied the waters
throughout these early periods of time. Not less
varied are the forms of palate bones and teeth,
in the same formations that contain these
spines; but as the cartilaginous skeletons to
which they belonged have usually perished,
and the teeth and spines are generally dispersed,
it is chiefly by the aid of anatomical
analogies, or from occasional juxtaposition in
the same stratum, that their respective species
can be ascertained.
Fossil Rays.
The Rays form the fourth family in the order
Placoidians. Genera of this family abound
among living fishes; but they have not been
found fossil in any stratum older than the Lias;
they occur also in the Jurassic limestone.
Throughout the tertiary formation they are
very abundant; of one genus, Myliobatis, there
are seven known species ; from these have been
derived the palates that are so frequent in the
London clay and crag. (See PI. 27d, B. Fig.
14.) The genus Trygon, and Torpedo, occur
also in the Tertiary formations.
attached to strong muscles; but articulate with a bone beneath
them. The Spine of Balistes also is kept erect by a second
spine behind its base, acting like a bolt or wedge, which is
simultaneously inserted, or withdrawn, by the same muscular
motion that raises or depresses the spine.