530 HISTORICAL PALAEONTOLOGY. 



Polyzoa. The most important genera of Brachiopods are 

 Spirifera, Producta, Strophalosia, Camarophoria, and Lingula. 

 Bivalves are moderately numerous, the commonest forms be- 

 longing to the family Trigoniada and to the genus Axinus 

 (Schizodus). Other forms belong to the genera Mytilus, Bake- 

 wellia, and Avicula. Gasteropods and Cephalopods are, upon 

 the whole, very poorly represented in the Permian series. 



The most important Permian fossils are referable to the 

 Vertebrata, Fishes are comparatively very abundant, and be- 

 long almost entirely to the order of the Ganoids. The most 

 characteristic genera are Pal&oniscus, Platysomus, Pygopterus, 

 Gyropristis, Acrolepis, and Ccelacanthus. The Amphibians are 

 represented by various forms belonging to the Labyrinthodontia. 

 True Reptiles are represented by the Protorosaurus of the 

 " Kupfer-schiefer " of Germany, and somewhat doubtfully by 

 the Chelonian footprints (Chelichnus) from the Permian Sand- 

 stones of Dumfriesshire. 



CHAPTER LI. 



TRIASSIC PERIOD. 



ROCKS OF THE PERIOD. 



WE come now to the consideration of the great Mesozoic, or 

 Secondary series of formations, consisting, in ascending order, 

 of the Triassic, Jurassic, and Cretaceous systems. The Trias- 

 sic group forms the base of the Mesozoic series, and corre- 

 sponds with the higher portion of the New Red Sandstone of 

 the older geologists. Like the Permian Rocks, and as implied 

 by its name, the Trias admits of a subdivision into three 

 groups a Lower, Middle, and Upper Trias. Of these sub- 

 divisions the middle one is wanting in Britain ; and all have 

 received German names, being more largely and typically de- 

 veloped in Germany than in any other country. Thus, the 

 Lower Trias is known as the Bunter Sandstein; the Middle 

 Trias is called the Muschelkalk, and the Upper Trias is known 

 as the Keuper. 



I. The lowest division of the Trias is known as the Bunter 

 Sandstein, from the generally variegated colours of the beds 

 which compose it (German, bunt, variegated). The Bunter 

 Sandstein of the continent of Europe consists of red and 



