These scales may overlap; or they may be rhomboidal
plates, placed edge to edge in oblique rows; or they have the form
of large-sized bony plates, which are commonly united in the region
of the head to form a regular buckler. (3) The _Placoid Fishes_,
or _Elasmobranchii_, comprising the Sharks, Rays, and _Chimoeroe_
of the present day, in which the skeleton is cartilaginous; the
tail is unsymmetrically lobed; and the scales have the form of
detached bony plates of variable size, scattered in the integument.
It is to the two last of these groups that the Devonian fishes
belong, and they are more specially referable to the _Ganoids_.
The order of the Ganoid fishes at the present day comprises but
some seven or eight genera, the species of which principally or
exclusively inhabit fresh waters, and all of which are confined
to the northern hemisphere. As compared, therefore, with the Bony
fishes, which constitute the great majority of existing forms,
the Ganoids form but an extremely small and limited group. It was
far otherwise, however, in Devonian times. At this period, the
bony fishes are not known to have come into existence at all, and
the Ganoids held almost undisputed possession of the waters. To
what extent the Devonian Ganoids were confined to fresh waters
remains yet to be proved; and that many of them lived in the sea
is certain.
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