The material upon which this study is based belongs to the Reinbek formation, and is present in the collections of the Rijksmuseum van Geologie en Mineralogie, Leiden. Of each sample a counting-list of the molluscs is given; of these the features they have in common are given in general account. Three groups of borings can be distinguished with regards to the species content in the basal layers; especially the graph of Astarte radiata in the basal part of the boring is important. Probably these groups have a correlation with differences in habitat. The different sediments belong to one sedimentation cycle; the gradual decline of the basin caused a gradual change in the species content. A thanatocoenosis that is found in the more western borings at the base, is present in the eastern borings higher in the section. One can recognize formations that are distinguished by other authors on the base of molluscs, but these do not appear to be separated by boundaries in the species content. The soft concretions in situ are linked to a certain thanatocoenosis, but the 50% clay — 50% sand boundary appears to vary. The miocene sea must have extended farther eastwards; there are still some miocene deposits left in places with salt-dome activity. Species typical for the “Hemmoor Stufe” are also found in sediments of the Laag van Ticheloven; the sediments of the former formation are here regarded as a shellow water facies of the Reinbek Stufe. The most important species, which can be used to characterise the thanatocoenosis, are, from the base to the top: Hiatella arctica, Astarte radiata, Spisula spec., and Limopsis aurita.