Study of Palaeoperidinium pyrophorum and P. cretaceum leads to the conclusion that the principal fossilized wall in both species (here termed the exophragm) formed in an extrathecal position in intimate contact with the exterior surface of the theca. Specimens of P. pyrophorum from ten widespread Late Cretaceous and Paleocene localities and of P. cretaceum from the Early Cretaceous of Alaska were examined with light and scanning electron microscopy. An electronic technique for inverting an SEM image (i.e., exchanging light for dark pixels) and a manual microcasting technique provide support for this interpretation. In some specimens, the exophragm encloses a two-walled intrathecal cyst, like the conventional periphragm and endophragm of many fossil peridiniacean dinoflagellates. A minute but elaborately configured structure in a midventral position on the exophragm apparently includes the insertion points of the two flagella; it is illustrated with SEM stereopairs. Pillar-like projections produced where material of the developing cyst wall partially filled trichocyst pores of the theca are especially distinctive features on the interior surface of the exophragm. Comparable structures occur on the exterior of the periphragm in some species of other genera (e.g., Deflandrea, Subtilisphaera, Mikrocysta, and Phanerodinium), where they appear to have originated similarly; that is, from material that formed the periphragm. Such an interpretation accords with the intrathecal position generally attributed to the periphragm. However, the conclusion that the pillar-like structures interconnecting the two walls in still other dinoflagellates (e.g., certain species of Charlesdowniea, Chlamydophorella, Gardodinium, and Dinopterygium) are also trichocyst pore fillings has an unexpected implication: the outer-most wall in many species of fossil dinoflagellates is an exophragm comparable to that in Palaeoperidinium. The further critical study of the exophragm that this interpretation invites may lead to a better understanding of homologies among cyst wall structures and to a more consistent descriptive terminology for cysts with multiple walls. Concepts of the genus, Palaeoperidinium, and two species, P. pyrophorum and P. cretaceum, are emended to accord with observations in the SEM and the interpretation of their principal wall as an exophragm. Nineteen other species--18 listed as species of Palaeoperidinium by Williams et al. (1998) and P. eurypylum, which is reattributed herein to Palaeoperidinium from Saeptodinium--are provisionally accepted in the genus pending clarification of their characters in the SEM.

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