This article deals with the occurence of a peculiar group of fossil corals, called Heliolitida. They can especially be found in the Red Saalian boulder clay of the Hondsrug, between the towns Groningen and Haren, in the northern part of the Netherlands. They have been transported by glaciers from ordovician and silurian limestone-deposits in the Baltic area. Investigation of the available bouldermaterial showed that ten different genera could be distinguished, i.e. Propora M.EDW.- H ., 1849; Plasmoporella KIÄR, 1897; Plasmopora M. EDW.-H ., 1849; Coccoseris EICHWALD, 1855; Acidolites LANG-S.-T ., 1940; Protaraea M. EDW.-H., 1851; Proheliolites KIÄR, 1897; Heliolites DANA , 1849; Stelliporella WENTZEL, 1895; Podollites BONDARENKO, 1971. Most of the boulders, however, belong to only two genera, i.e. Heliolites DANA , and Plasmoporella KIÄR. The systematic position of the Heliolitida is shortly discussed. Because of a few features, such as the presence of coenenchyme, the 12 rows of septal spines in each corallite and the coenenchymal mode of budding, the Heliolitida most likely form an independent group of the Anthozoa, apart from the Tabulata. In some aspects they are regarded as precursors of the recent Octocorallia. Some remarks are dedicated to the fact that there occur two main types of coenenchyme within the Heliolitida, i.e. a tubular and a vesicular one. Statistically could be established that in the genera with the vesicular coenenchyme, the corallites are far more crowded than in the genera with a tubular one. The suggestion has been made that there excists a relation between the mutual corallitedistances and the tijpe of coenenchymal tissue. In Plasmopora M. EDW. H. a change of the vesicular coenenchyme can be noticed. The corallites in this genus are far less crowded than in Propora M. EDW.H. and in Plasmoporella KIÄR. The vesicular tissue there fore had to fill up more space between the corallites. In Plasmopora M.EDW.H the separate chalcbladders become more or less arranged in vertical rows, resembling tubules. In transverse section these tubules are subpolygonal in outline. Probably because of the larger intercorallite distances the organism had to change the original vesicular coenenchymal tissue into a more tubular one. It is known that such a construction is much stronger. Though the different build up of the corallum of Plasmopora most likely is caused by ecological circumstances, boulders of this genus are quite unfit to give any further detailed information.