270 Murchison on the Permian System. 



genus. The Avicula is also a good Permian shell, the A. Kazanensis 

 being the best type in Russia, whilst the A. antiqua is there com- 

 mon to this deposit and the carboniferous. 



The Gastoropods, so abundant in the carboniferous era have un- 

 dergone great diminution before the formation of the Permian stra- 

 ta, and had great difficulty in accommodating themselves to new 

 conditions ; still more so the Cephalopods, for the forms of Gonia- 

 tites, Nautili, and Orthoceratites, so very common in the preceding 

 epoch, are almost unknown in this system, a fragment or two of 

 one genus (Nautilus ?) alone having been found in all parts of 

 Europe. This scarcity of Cephalopods at the close of the Palaeozoic 

 series has a remarkable parallel in a subsequent geological period ; 

 for as these animals were reproduced in vast abundance and under 

 many new forms in the Triassic, Jurassic, and Cretaceous systems, 

 so towards the termination of the last of these we perceive a second 

 and similar disappearance of the greater number of the shelly Cephalo- 

 pods. The extreme reduction of the Gasterpods at the close of the 

 cretaceous periods, as indicated by M. Alcide D'Orbigny, is also 

 pointed out as an additional feature of analogy to the Permian changes. 

 Trilobites v so eminently characteristic of the Silurian system, and 

 which dwindle away to a few small species in the carboniferous sys- 

 tem, are unknown in the Permian of Western Europe, and in Russia 

 are only represented by a species of Limulus. Fishes, on the other hand, 

 are numerous in proportion to the other Permian classes, 43 or 44 

 species being named, and several from Russia being yet undescribed ; 

 these are all, with one exception, absolutely peculiar to the stratum 

 in which they occur, thus confirming the truth of the generalisation 

 of Agassiz, that these vertebrata mark with great precision the age 

 of the stratum in which they are found. Lastly, the Permian beds 

 of Russia, like the Dolomitic conglomerate of England and the 

 Kupferschiefer of Germany, contain bones of the codont Saurians, in- 

 dicating the earliest appearance of animals of that high organisation, 

 and their direct association with Palaeozoic shells and plants, some of 

 which are undistinguishable from true carboniferons species. 



After thus following it back in time, the Permian fauna is next 

 considered in horizontal extension or distance, the fossils of Russia 

 being compared with those of similar age in western Europe. The 



