482 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



simply state some broad facts with which I became better ac- 

 quainted on my last visit to St. Petersburg. Previously, indeed, 

 to that visit, my colleagues and myself had convinced ourselves 

 that all or nearly all the Silurian deposits in the governments of 

 St. Petersburg and Reval belonged to the Lower Silurian group 

 only, an inference derived both from a more correct examination 

 of these fossils than had been made when the earlier communica- 

 tions on this subject were made to the Geological Society (Proceed- 

 ings, vol. iii. p. 398. 717.,) and from a better acquaintance with 

 the Scandinavian palasozoic succession, which, in reference to the 

 lowest group, is of great importance, by exhibiting an inter- 

 mixture of well-known English Lower Silurian types with forms 

 abundantly common to that country and Russia. In this manner, 

 though the British typical species thin out and decrease in num- 

 ber, as the deposits are followed from west to east, their place in 

 the series is distinctly and unequivocally preserved. 



As a whole, then, I now beg to state, that the Silurian deposits of 

 the Baltic governments of Russia, which consist in ascending order 

 of (1.) clay, (2.) Ungulite sandstone, (3.) bituminous schist, and 

 (4.) Pleta or Orthoceratite limestone, unquestionably constitute the 

 very same group as that which has been shown to be Lower Silu- 

 rian on the main land of Sweden ; for amid certain new forms they 

 are charged with the same characteristic fossils — Asaphus ex- 

 pansus, Ulcenus crassicauda, Orthoceratites duplex, Echino- 

 sphcerites in abundance, and the coral Chcetetes petropolitanus. 

 When, however, we enter into details, there are considerable 

 variations both in mineral and zoological development. Thus, 

 whilst in Sweden the lowest stratum is a sandstone, in Russia it is 

 a shale of great thickness ; though in both tracts the analogy is 

 preserved by the lowest band containing fucoids only. The second 

 Russian stratum or Ungulite sandstone, is peculiar to Russia in 

 containing that remarkable horny shell, the Obolus of Eichwald or 

 Unguolite of Pander, but the overlying pleta limestone or great 

 centre of animal remains is full of the characteristic shells of the 

 lower limestone of Scandinavia. And here it is curious to observe, 

 how in receding from our typical regions in Britain certain generic 

 and specific forms gradually disappear, though the families of 

 fossils remain the same. Thus the two most common of the- 

 Lower Silurian trilobites of England, the Asaphus Buchii and 

 Asaphus tyrannus, which are not unfrequent in Norway, begin to 

 be scarce in Sweden, and in Russia have only been very rarely 

 discovered,* The Trinucleus, so very prolific in Great Britain, 



* Professor Eichwald had previously observed that the Asaphus dilatatus, 

 which is, we conceive, identical with the trilobite called Asaphus Buchii of the 

 British Isles, occurs at Oduisholm, an isle adjacent to Esthonia. More recently 

 H.I.H. the Duke of Leuchtenberg has found both the A. Buchii and A. 

 tyrannus in the quarries of Grafskaya Slavenka, to the south of Czarskoe 

 Celo, where they are associated with many other typical Lower Silurian fossils, 

 and some new genera and species which the Prince has named. ( See " Be- 

 schreibung einer neuen Thierreste aus den silurischen Kalk-sehichten von 

 Tzarskoje Celo von Maximilian Herzog von Leuchtenberg.") 



