Periostracum

Rarity: Common

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At a glance

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Notion

Domain

Science and technology

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What Wikipedia says

The periostracum ( PERR-ee-OS-trə-kəm) is a thin, organic coating (or "skin") that is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods.

Excerpt from the Wikipedia article “Periostracum”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: H. Zell, CC BY-SA 3.0, via Wikimedia Commons.