116 GEOLOGICAL MEMOIRS. 



have undergone, one is struck by the simultaneity of the principal 

 phaenomena to which they have been subject, namely the appear- 

 ance and extinction of species. In spite of the differences presented 

 by the almost horizontal plains of Russia, when compared with the 

 countries further to the west, the succession of species has taken 

 place throughout in the same order. The lower members of the 

 Silurian system, for example, are there characterized, as in all other 

 countries hitherto investigated, by the abundance of Orthis^ Lep- 

 tcena, Orthoceratites and Trilobites^ while the upper beds afford a 

 multitude of corals, as Catenipora and Favosites. The Devonian 

 rocks there, as in Scotland, exhibit a remarkable development of 

 the class of fishes, and, as in Devonshire, the Productus first ap- 

 pears and the Spirifer first becomes abundant in rocks of this age. 

 Most of the species which formed the submarine population during 

 these two earlier periods becoming one after another extinct in 

 Russia as elsewhere, they are there also replaced by others, amongst 

 which the Productus everywhere characteristic of the carbonife- 

 rous rocks is the most striking. And lastly, the Permian deposits, 

 although deposited beneath the waters of a far more widely extended 

 ocean than the contemporaneous beds of Western Europe, still pre- 

 sent a remarkable agreement in the appearance of Saurians, and the 

 complete extinction of Trilobites, Goniatites, Orthoceratites and 

 Bellerophons, of which we nowhere find traces. If, struck by this 

 strange sequence, we turn our attention to North America, and there 

 discover a series of analogous phsenomena, it will appear certain that 

 all these modifications of species, their extinction and the introduc- 

 tion of new ones, cannot be owing to mere change in marine cur- 

 ,rents or other causes more or less local and temporary, but depend 

 on general laws which govern the whole animal kingdom. 



On comparing together the four divisions of the palaeozoic series 

 as developed in Russia, we find that the number of species gradually 

 increases from the Silurian to the Carboniferous in a constant ratio 

 analogous to that observed in the general fauna of the epoch. With 

 regard to the Permian division, the gradual advance seems there to 

 have received a decided check, and, as is the case elsewhere, the 

 number of species is much less considerable than during the earlier 

 period. 



If in the next place we compare the species of each system or 

 division of the whole period, it is hardly possible not to be struck 

 with the fact, that in a district where the deposits appear to have 

 been uninterrupted and continuous, there are so few species passing 

 from one system into another. Eight species only are common to 

 the beds of two of these great divisions, two seem to have endured 

 the vicissitudes of a greater number, while Chonetes sarcinulata is 

 the only one that appears to have survived throughout the palaeozoic 

 period. When however in this kind of comparative view we include 

 a greater area, and extend our observation for example to the whole 

 of Europe, the number of species common to several systems sensi- 

 bly increases, and we then discover relations between the thickness 

 of the deposits through which the species extend and the geographi- 



