ABSTRACT This report presents details of the development of the Silurian-Devonian boundary in the Barrandian area in Central Bohemia. It begins with a summary of previous investigations, followed by a survey of the general facies development of the region. The most important stratigraphic sections in which the boundary occurs are described and illustrated. Details of biostratigraphy concentrate primarily on the occurrence of index fossils. The second part of the paper is concerned with a discussion of individual faunal groups and their importance in delineating the boundary, their dependence on facies, and their value in correlation. Finally, the general development of the Silurian-Devonian boundary in the Barrandian is discussed, the stage and zonal subdivisions of the boundary beds are revised, and stratotypes are defined or specified in greater detail. Principal conclusions may be summarized as follows: 1. The Silurian-Devonian boundary is defined at the base of the Monograptus uniformis Zone which is coincident with the Pridolian-Lochkovian boundary. 2. Although unaccompanied by facies change, the boundary is well defined bio-stratigraphically on the basis of graptolites and other faunas. 3. Recent investigations confirm that there is no demonstrable stratigraphic hiatus or facies change at the boundary, which can be defined confidently only through biostratigraphy. In the vicinity of the boundary, however, mainly in the Lower Lochkovian, sedimentation was locally condensed and a relatively long time-interval may be represented by a small thickness of sediments. 4. The base of the Devonian is marked by the sudden appearance of Monograptus uniformis. Two subspecies, M. uniformis uniformis and M. uniformis angustidens, appear simultaneously or separated only by a very short interval. 5. The first occurrence of the trilobite Warburgella rugulosa rugosa is a suitable auxiliary indicator of the boundary, and is a prominent marker horizon in the Lower Lochkovian. It appears essentially simultaneously with M. uniformis. Because of its wider facies tolerance, the species is used for correlating between platy limestones and shales and the purely carbonate development. 6. The boundary can also be identified by using faunas that occur in the upper part of the Pridolian. These include: abundant Monograptus transgrediens, Dayia bohemica (particularly in the carbonate facies), Spathognathodus steinhornensis eosteinhornensis, and Boucia ornatissima (in the shale-rich development). The trilobite Prantlia (Tetinia) minuta is a significant horizon marker at the Pridolian-Lochkovian boundary in the carbonate facies. In addition to graptolites, trilobites, and conodonts (especially Icriodus woschmidti), the sudden increase of an ostracod association including Acanthoscapha bohemica serves to identify the lowest Lochkovian in the platy-limestone facies. The latter association, however, may begin in the latest Pridolian. 7. Accumulations of remains occur at various stratigraphic horizons within the Pridolian and Lower Lochkovian. They reach a maximum in the platy-limestone and shale facies in the southeast limb of the Barrandian in the basal Lochkovian, where they are referred to as the Scyphocrinites Horizon. The occurrence of the genus cannot, therefore, be used to delineate the Silurian-Devonian boundary or to define the stratigraphic units: Budnanian, Pridolian, or Lochkovian. End_Page 104------------------------ 8. Recent work has shown that almost all faunal groups in the boundary beds are facies-controlled. The fauna of the platy limestones and shales differs markedly from that of the pure carbonate facies. 9. The following chronostratigraphic division may be recognized: the Budnanian (equals Upper Silurian) Series is divided into the Ludlovian and Pridolian stages; the Lochkovian is the earliest Devonian stage. 10. The Pridolian-Lochkovian or Budnanian-Lochkovian boundary is redefined on the basis of graptolites, and stratotypes are revised accordingly.