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 Palæozoic 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 Palæozoic 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 Palæozoic 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). The _Amphibians_ of the Permian period belong principally to the order of the _Labyrinthodonts_, which commenced to be represented in the Carboniferous, and has a large development in the Trias. Under the name, however, of _Paloeosiren Beinerti_, Professor Geinitz has described an Amphibian from the Lower Permian of Germany, which he believes to be most nearly allied to the existing "Mud-eel" (_Siren lacertina_) of North America, and therefore to be related to the Newts and Salamanders (_Urodela_). [Illustration: Fig. 138.--_Protorosaurus Speneri_, Middle Permian, Thuringia, reduced in size. (After Von Meyer.) [Copied from Dana.]]