Of the latter, the most important are
species belonging to the genera _Megalichthys_ and _Rhizodus_,
comprising large fishes, with rhomboidal scales, unsymmetrical
("heterocercal") tails, and powerful conical teeth. These fishes
are sometimes said to be "sauroid," from their presenting some
Reptilian features in their organisation, and they must have been
the scourges of the Carboniferous seas. The remains of _Placoid_
fishes in the Carboniferous strata are very numerous, but consist
wholly of teeth and fin-spines, referable to forms more or less
closely allied to our existing Port Jackson Sharks, Dog-fishes,
and Rays. The teeth are of very various shapes and sizes,--some
with sharp, cutting edges (_Petalodus, Cladodus_, &c.); others in
the form of broad crushing plates, adapted, like the teeth of the
existing Port Jackson Shark (_Cestracion Philippi_), for breaking
down the hard shells of Molluscs and Crustaceans. Amongst the many
kinds of these latter, the teeth of _Psammodus_ and _Cochliodus_
(fig. 131) may be mentioned as specially characteristic. The
fin-spines are mostly similar to those so common in the Devonian
deposits, consisting of hollow defensive spines implanted in
front of the pectoral or other fins, usually slightly curved,
often superficially ribbed or sculptured, and not uncommonly
serrated or toothed.
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