peri·​os·​tra·​cum | \ ˌperēˈästrəkəm\
plural periostraca\ -​kə \

Definition of periostracum

: a chitinous layer covering the exterior of the shell in many mollusks, being usually well developed in freshwater forms, and serving to protect the shell from corrosion

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History and Etymology for periostracum

New Latin, from peri- + Greek ostrakon shell

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