# periostracum

> English word · Noun

## Definitions
1. The proteinaceous outer sheath of many molluscan shells.

## Etymology
Literally "surrounding shell", from peri- + ostracum.

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/word/periostracum