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]

The Collaborative International Dictionary of English v.0.48:

Epidermis \Ep`i*der"mis\, n. [L., fr. Gr. ?; ? over + ? skin,
   fr. ? to skin. See Tear, v. t.]
   1. (Anat.) The outer, nonsensitive layer of the skin;
      cuticle; scarfskin. See Dermis.
      [1913 Webster]

   2. (Bot.) The outermost layer of the cells, which covers both
      surfaces of leaves, and also the surface of stems, when
      they are first formed. As stems grow old this layer is
      lost, and never replaced.
      [1913 Webster]

WordNet (r) 3.0 (2006):

epidermis
    n 1: the outer layer of the skin covering the exterior body
         surface of vertebrates [syn: epidermis, cuticle]