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 introduced to the deep-sea accumulations of the
same period. Nor can the problem at issue be solved by an appeal
to the phenomena of the British area alone, be the testimony of
these what it may.
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