[Illustration: Fig. 136.--_Arca antiqua_. Permian.]
The _Univalves_ (_Gasteropoda_) are rare, and do not demand special
notice. It may be observed, however, that the Palaeozoic genera
_Euomphalus, Murchisonia, Loxonema_, and _Macrocheilus_ are still
in existence, together with the persistent genus _Pleurotomaria_.
_Pteropods_ of the old genera _Theca_ and _Conularia_ have been
discovered; but the first of these characteristically Palaeozoic
types finally dies out here, and the second only survives but a
short time longer. Lastly, a few _Cephalopods_ have been found,
still wholly referable to the Tetrabranchiate group, and belonging
to the old genera _Orthoceras_ and _Cyrtoceras_ and the long-lived
_Nautilus_.
[Illustration: Fig. 137.--_Platysomus gibbosus_, a "heterocercal"
Ganoid, from the Middle Permian of Russia.]
Amongst _Vertebrates_, we meet in the Permian period not only
with the remains of Fishes and Amphibians, but also, for the
first time, with true Reptiles. The _Fishes_ are mainly _Ganoids_,
though there are also remains of a few Cestraciont Sharks. Not
only are the _Ganoids_ still the predominant group of Fishes, but
all the known forms possess the unsymmetrical ("heterocercal")
tail which is so characteristic of the Palaeozoic Ganoids. Most
of the remains of the Permian Fishes have been obtained from the
"Marl-slate" of Durham and the corresponding "Kupfer-schiefer" of
Germany, on the horizon of the Middle Permian; and the principal
genera of the Ganoids are _Paloeoniscus_ and _Platysomus_ (fig.
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