It was formerly supposed that the Old Red Sandstone
of Scotland and Ireland, with its abundant fish-remains, might
perhaps be a fresh-water deposit, since the habitat of its fishes
is uncertain, and it contains no indubitable marine fossils. It
has been now shown, however, that the marine Devonian strata
of Devonshire and the continent of Europe contain some of the
most characteristic of the Old Red Sandstone fishes of Scotland;
whilst the undoubted marine deposit of the Corniferous limestone
of North America contains numerous shark-like and Ganoid fishes,
including such a characteristic Old Red genus as _Coccosleus_.
There can be little doubt, therefore, but that the majority of
the Devonian fishes were truly marine in their habits, though
it is probable that many of them lived in shallow water, in the
immediate neighbourhood of the shore, or in estuaries.
[Illustration: Fig. 102.--Fishes of the Devonian rocks of America.
a, Diagram of the jaws and teeth of _Dinichthys Hertzeri_,
viewed from the front, and greatly reduced; b, Diagram of the
skull of _Macropetalichthys Sullivanti_, reduced in size; c,
A portion of the enamelled surface of the skull of the same,
magnified; d, One of the scales of _Onychodus sigmoides_, of
the natural size; e, One of the front teeth of the lower jaw of
the same, of the natural size: f, Fin-spine of _Machoeracanthus
major_, a shark-like fish, reduced in size.
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