Abstract Dinoflagellates, their planktonic nature, world-wide distribution, and their geologic record from the Permian, are reviewed. Re-interpretation of their morphology indicates that in fossil forms which appear to show the plate-like arrangement and the sulcus and groove of the theca of the motile stage of dinoflagellates, this is merely a reflection of those structures on the walls and outward-projecting processes of an internal cyst. The walls of the cyst itself may lie close to the thecal walls in some forms and thus faithfully reproduce the features of plates and grooves. In others, the cyst appears to occupy a small space in the center of the thecal cavity and is supported against the theca by spines or other structures. Some dinorlagellates are short-ranging, but widespread, and have sufficient distinctive features to make them valuable stratigraphie markers. Paleoecology of these entities is not yet well known.