). The _Brachiopoda_ are also represented
in moderate numbers in the Permian. Along with species of the
persistent genera _Discina, Crania_, and _Lingula_, we still
meet with representatives of the old groups _Spirifera, Athyris_,
and _Streptorhynchus_; and the Carboniferous _Productoe_ yet
survive under well-marked and characteristic types, though in
much-diminished numbers. The species of Brachiopods here figured
(fig. 135) are characteristic of the Magnesian Limestone in Britain
and of the corresponding strata on the Continent. Upon the whole,
the most characteristic Permian _Brachiopods_ belong to the genera
_Producta, Strophalosia_, and _Camarophoria_.

The _Bivalves_ (_Lamellibranchiata_) have a tolerably varied
development in the Permian rocks; but nearly all the old types,
except some of those which occur in the Carboniferous, have now
disappeared. The principal Permian Bivalves belong to the groups
of the Pearl Oysters (_Aviculidoe_) and the _Trigoniadoe_,
represented by genera such as _Bakewellia_ and _Schizodus_; the
true Mussels (_Mytilidoe_), represented by species which have
been referred to _Mytilus_ itself; and the Arks (_Arcadoe_),
represented by species of the genera _Arca_ (fig. 136) and
_Byssoarca_. The first and last of these three families have a
very ancient origin; but the family of the _Trigoniadoe_, though
feebly represented at the present day, is one which attained
its maximum development in the Mesozoic period.

[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