For the purpose of defining the area and borders of former Littorina-sea (littorine stage of development of the Baltic) in Finland, the subfossil diatoms are a proof. During the year 1916 there were clay samples collected among the river Pyhäjoki, from the places that are presumably above and below the Littorina sea border. The examination of the samples showed that at six of them there contained no fossils, and four of them had only one kind of them. After this pre-examination 46 samples more were collected from the similar spots than the ones where the diatom fossils were found. This article presents the preliminary results of the project defining the borders of the Littorina-sea. The places of discovery of the diatom fossils are described.
The fossils in Ostrobothnia are rich of diatom flora. The most commonly occurred species were e.g. Epithemia turgida and Gyrosigma attenuatum. The half of the total number of the species are found in only five samples; most species feature only in few samples, if not in only one sample.
The article presents minimum levels of Littorina-sea as preliminary results.