from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The hard chitinous outer covering of the shell of many mollusks, especially freshwater ones, that protects the shell from the erosive action of water.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The proteinaceous outer sheath of many molluscan shells.
from the GNU version of the Collaborative International Dictionary of English
- n. A chitinous membrane covering the exterior of many shells; -- called also epidermis.
from The Century Dictionary and Cyclopedia
- n. The horny epidermal investment of the shells of most mollusks.
New Latin : peri- + Greek ostrakon, shell; see ost- in Indo-European roots.(American Heritage® Dictionary of the English Language, Fourth Edition)
Literally "surrounding shell," from peri- + ostracum. (Wiktionary)