s_. These are
numerous; but, with the exception of the still dubious "Conodonts,"
belong wholly to the groups of the _Ganoids_ and the _Placoids_
(including under the former head remains which perhaps are truly
referable to the group of the _Dipnoi_ or Mud-fishes). Amongst the
_Ganoids_, the singular buckler-headed fishes of the Upper Silurian
and Devonian (_Cephalaspidoe_) have apparently disappeared; and
the principal types of the Carboniferous belong to the groups
respectively represented at the present day by the Gar pike
(_Lepidosteus_) of the North American lakes, and the _Polypterus_
of the rivers of Africa. Of the former, the genera _Paloeoniscus_
and _Amblypterus_ (fig. 130), with their small rhomboidal and
enamelled scales, and their strongly unsymmetrical tails, are
perhaps the most abundant. 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. The genera _Ctenacanthus, Gyracanthus,
Homacanthus_, &c., have been founded for the reception of these
defensive weapons, some of which indicate fishes of great size
and predaceous habits.

[Illustration: Fig. 131.--Teeth of _Cochliodus contortus_.
Carboniferous Limestone, Britain.]

[Illustration: F