'Periostracum' definitions:

Definition of 'Periostracum'

From: GCIDE

Periostracum \Per`i*os"tra*cum\, n.; pl. Periostraca. [NL., fr. Gr. peri` around + ? shell of a testacean.] (Zool.) A chitinous membrane covering the exterior of many shells; -- called also epidermis. [1913 Webster]