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Description of PeridiniaceaeMotile cells vary in shape from subspherical, ovoidal, oblate spheroids, to fusiform. Distinct apical and 2 antapical horns. Tabulation comprises the following pattern: 4 apical plates; 3 anterior intercalary plates; 7 precingular plates; 5 postcingular plates; and 2 antapical plates. Form cysts. Fossilized cysts are organic-walled, calcareous, or siliceous.