Abstract
Summary: Assemblages of microplankton are described from the Lower Calcareous Grit, Hambleton Oolite, Osmington Oolite and Upper Calcareous Grit of Yorkshire. They consist of dinoflagellates and hystrichospheres. The hystrichospheres include new species of Micrhystridium and Pterospermopsis, and five species are described for the first time from Britain. Five species of dinoflagellates are described from Britain for the first time also. The stratigraphic distribution of species present is summarized.

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