105.--A, _Polypterus_, a recent Ganoid
fish; B, _Osteolepis_, a Devonian Ganoid; a a, Pectoral fins,
showing the fin-rays arranged round a central lobe.]
[Illusration: Fig. 106.--_Holoptychius nobilissimus_, restored.
Old Red Sandstone, Scotland. A, Scale of the same.]
Leaving the Ganoid fishes, it still remains to be noticed that
the Devonian deposits have yielded the remains of a number of
fishes more or less closely allied to the existing Sharks, Rays,
and _Chimoeroe_ (the _Elasmobranchii_). The majority of the forms
here alluded to are allied not to the true Sharks and Dog-fishes,
but to the more peaceable "Port Jackson Sharks," with their blunt
teeth, adapted for crushing the shells of Molluscs. The collective
name of "Cestracionts" is applied to these; and we have evidence of
their past existence in the Devonian seas both by their teeth, and
by the defensive spines which were implanted in front of a greater
or less number of the fins. These are bony spines, often variously
grooved, serrated, or ornamented, with hollow bases, implanted
in the integument, and capable of being erected or depressed
at will. Many of these "fin-spines" have been preserved to us
in the fossil condition, and the Devonian rocks have yielded
examples belonging to many genera. As some of the true Sharks
and Dog-fishes, some of the Ganoids, and even some Bony Fishes,
possess similar defences, it is often a matter of some uncertainty
to what group a given spine is to be referred.
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