Meaning of periostracum

per•i•os•tra•cum

Pronunciation: (per"ē-os'tru-kum), [key]
— pl. -ca
  1. the external, chitinlike covering of the shell of certain mollusks that protects the limy portion from acids.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.