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 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. 137).