The Collaborative International Dictionary of English v.0.48:

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]