The apical pore structures are described and classified from fifteen genera of marine dinoflagellates. A new terminology is proposed for the three types of thecal plate which may be involved: pore plate, cover plate, canal plate. In the genera Helgolandinium and Pyrophacus the pores are slit-like and only the pore plate is present. In some members of Gonyaulax the pore is hook-shaped and a simple cover plate is present, but in others the pore is tubular and lacks a cover plate. Most members of the genus Protoperidinium and its near relatives (e.g. Diplopsalis, Scrippsiella) have a tubular pore and a canal plate is present. In Pyrodinium where there is no canal plate but an elaborate cover plate is present, the ornamentation and shape of the cover plate is rather variable.