84 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



(26 in number) the Modiola is very characteristic of the Permian 

 system, both in Russia and England ; and though the large species 

 of Axinus so well known in England has not yet been found in 

 Russia, its place is there taken by two other species of the same 

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

 nensis being the best type in Russia, whilst the A. antiqua is 

 there common to this deposit and the carboniferous. 



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

 dergone great diminution before the formation of the Permian 

 strata, and have had great difficulty in accommodating themselves 

 to new conditions ; still more so the Cephalopods, for the forms of 

 Goniatites, Nautili, and Orthoceratites, so very common in the pre- . 

 ceding 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 Pa- 

 laeozoic 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 sys- 

 tems, so towards the termination of the last of these we perceive a 

 second and similar disappearance of the greater number of the shelly 

 Cephalopods. The extreme reduction of the Gasteropods at the 

 close of the cretaceous period, as indicated by M. Alcide D'Orbigny, 

 is also pointed out as an additional feature of analogy to the Per- 

 mian changes. Trilobites, so eminently characteristic of the Si- 

 lurian system, and which dwindle away to a few small species in 

 the carboniferous system, are unknown in the Permian of Western 

 Europe and in Russia, and 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 thecodont Sau- 

 rians, indicating 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 carboni- 

 ferous 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 

 number of species collected by the authors in Russia is 53 or about 

 one third of the total number of the whole European fauna of the 

 period, and of these 32 are peculiar to Russia, a large number 

 when the recency and rapidity of the survey of the authors is ad- 

 verted to ; and when it is considered that 33 species only were found 

 by Professor Sedgwick in deposits of this age in England, and 41 



