The article analyses a complex of the Ordovician's orthida (brachiopoda) of the lost outcrops on the river Pulkovka in Leningrad region. Some samples from a large collection of brachiopods in the holdings of the Mining Museum are analysed. These samples were collected in the XIX century from some outcrops on the river Pulkovka. The geological sections there were unique outcrops due to a stratigraphic interval from the Lower Cambrian to Middle Ordovician found in them. Many early Palaeozoic marine inverte-brates, such as brachiopods, cystoideas, bolboporits, pelecypods, gastropods, were collected from these sediments. In the middle of the XX century the outcrops on the river Pulkovka were lost as a result of landsliding processes. The authors have identified an orthida's complex in the collection, comprising 12 species. The structure of the complex indicates that the stratigraphic interval, from which brachiopods were collected, can be defined as Volkhov, Kund, Azery horizons of the Middle Ordovician.
At the present time researchers are considering recurrence (return of fauna and flora after the return of physical and geographical conditions) one of the drawbacks of biostratigraphic methods. Comparing the features of complex brachiopods and pelecypods of the Ilmen and Bureg beds (middle Frasnian) Ilmen cliff, the author demonstrates the great complexity of the mechanism of recurrence, which depends not only on environmental but also on biological factors. Also a significant role in reducing the reliability of the analysis of systematic diversity of fossiles has biological and paleontological problem species diagnostics (distinction used by different researchers species criteria).
The revision of stratigraphical schemes is one of the urgent problems at present time. This article gives data on history of formation of stratigraphical schemes of the Middle Devonian of North-West of the East-European platform. Modern stratigraphy of terrigenous layers of Middle Devonian is considered. The special attention is given to a problem of the boundary of the Middle and Upper Devonian. On the example of particular outcrops of the Leningrad region the importance of rhythmic stratigraphy in the splitting and correlation of deposits of the Middle and Upper Devonian is shown.
