The Collaborative International Dictionary
Periostraca
Periostracum \Per`i*os"tra*cum\, n.; pl. Periostraca. [NL., fr. Gr. ? around + ? shell of a testacean.] (Zo["o]l.) A chitinous membrane covering the exterior of many shells; -- called also epidermis.
Wiktionary
periostraca
n. (plural of periostracum English)