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Pisidium

noun Pi·sid·i·um \ -ēəm \
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Definition of Pisidium

: a genus of nearly cosmopolitan minute freshwater bivalves (family Sphaeriidae) usually somewhat smaller than those of the genus Sphaerium and having the siphons united at their ends

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Origin and Etymology of pisidium

New Latin, diminutive of Latin pisum pea

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