ng 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. One of these spines,
belonging to the genus _Machoeracanthus_, from the Devonian rocks
of America, has been figured in a previous illustration (fig.
102, f).

In conclusion, a very few words may be said as to the validity of
the Devonian series as an independent system of rocks, preserving
in its successive strata the record of an independent system
of life. Some high authorities have been inclined to the view
that the Devonian formation has in nature no actual existence,
but that it is made up partly of beds which should be referred
to the summit of the Upper Silurian, and partly of beds which
properly belong to the base of the Carboniferous. This view seems
to have been arrived at in consequence of a too exclusive study
of the Devonian series of the British Isles, where the physical
succession is not wholly clear, and where there is a striking
discrepancy between the organic remains of those two members
of the series which are known as the "Old Red Sandstone" and
the "Devonian" rocks proper. This discrepancy, however, is not
complete; and, as we have seen, can be readily explained on the
supposition that the one group of rocks presents us with the
shallow water and littoral deposits of the period, while in the
other we are introduce