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peridinium

noun
peri·​din·​i·​um | \ ˌperəˈdinēəm\

Definition of peridinium

1 capitalized : the type genus of Peridiniidae comprising marine and freshwater dinoflagellates that are typically subspherical to ovoid in outline and that sometimes have the test prolonged into short horns — compare ceratium
2 plural peridiniums\ -​ēəmz\ or peridinia\ -​ēə\ : any flagellate of the genus Peridinium

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History and Etymology for peridinium

borrowed from New Latin, borrowed from Greek peridīn-, base of peridīneîn “to whirl, spin around” (from peri- peri- + dīneîn “to whirl, spin about”) + New Latin -ium -ium

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