This article deals with the occurence of a peculiar group of Ordovician and Silurian tabulate corals in the Saalian boulder clay of Groningen and Haren. They are called Halysitids, because of the chainlike arrangement of the corallites. Investigation of the available bouldermaterial showed that four genera occur. Two of them i.e. Eocatenipora HAMADA, 1957 and Cystihalysites TCHERNYCHEV, 1941, were fully new. Most common are boulders or boulderfragments of Catenipora LAMARCK, 1816 (ca. 59%), followed by specimens of Halysites FISCHER VON WALDHEIM, 1813 (ca. 23%). The rich, well preserved material of Cystihalysites, made some investigations possible. Such as to trace the asexual reproduction. It has shown that in Cystihalysites the vesicular tissue of the tubules played a very important part in this reproduction, otherwise than in Halysites, where tubular growth started not before the corallites had reached their adult size. In Cystihalysites a young corallite developped out of the distal vesicular tissue, that was formed in a very early stage by the previous corallite. The peculiar build up of the tubule in Cystihalisites is quite different of that in Halysites. It consist of very little, strongly convex chalcbladders, which resemble the dissepiments in the Rugosa. Laterally they vertically form one single row of bladders. The construction of the tubule shows clearly that there was less chalc needed than in Halysites. It is probably caused by ecological reasons. Fur ther palaeoecological investigation is necessary.