per·i·os·tra·cum 
(pĕr′ē-ŏstrə-kəm)
(pĕr′ē-ŏstrə-kəm)n. pl. per·i·os·tra·ca (-kə)
The tough horny or fibrous outer covering of the shell of many mollusks and brachiopods.
[New Latin : PERI- + Greek ostrakon, shell; see ost- in the Appendix of Indo-European roots.]