Thecal fine-structure in the dinoflagellate genera Prorocentrum and Exuviaella

The structure of the thecae, as seen in the electron microscope, are described for two species of Prorocentrum and five of Exuviaella. For the first time it is possible to distinguish between true pores and the spines and thin areas of wall which have previously been described as pores or poroids. Several distinct features such as the spines covering the valves, the ridged valve margin and special flagella pore structures which are found in only two of the Exuviaella species suggest that it may be necessary to create a new genus.