periostracum definition

  • noun:
    • The tough chitinous external covering regarding the shell of numerous mollusks, specifically freshwater ones, that shields the layer from erosive action of water.
    • The proteinaceous external sheath of many molluscan shells.
    • A chitinous membrane layer covering the outside of many shells; -- labeled as additionally epidermis.
    • The horny epidermal financial investment associated with the shells of all mollusks.

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