The dinoflagellate cyst genus Willeidinium gen. nov., from the Middle Jurassic of France, shows an unusual, fundamentally gonyaulacacean paratabulation pattern with a reduced number of epicystal paraplates. The most important features are the presence of only five precingular and three apical paraplates, and one of two anterior intercalary homologues reaching the apex. The fifth precingular and the third apical paraplates are extremely large. This is interpreted as a result of fusion of adjacent paraplates (5"+6" and 3'+4') in a gonyaulacacean paratabulation pattern with A/ai-arrangement. The obvious relationship of Willeidinium with other Middle Jurassic leptodinioidean dinoflagellate cysts required the emendation of the families Gonyaulacaceae and Ceratocoryaceae. The application of the plate labeling systems of Kofoid and Taylor-Evitt is discussed. The type species, W. baiocassinum sp. nov., seems to be a good stratigraphic marker for the latest Bajocian to early Bathonian of Europe.

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