Source : Webster's Revised Unabridged Dictionary (1913)

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}.