The Collaborative International Dictionary of English v.0.48:

Testaceous \Tes*ta"ceous\, a. [L. testaceus, fr. testa a shell.
   See Testa.]
   1. Of or pertaining to shells; consisted of a hard shell, or
      having a hard shell.
      [1913 Webster]

   2. (Bot. & Zool.) Having a dull red brick color or a brownish
      yellow color.
      [1913 Webster]

   Testaceous animals (Zool.), animals having a firm,
      calcareous shell, as oysters and clams, thus distinguished
      from crustaceous animals, whose shells are more thin and
      soft, and consist of several joints, or articulations, as
      lobsters and crabs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

testaceous
    adj 1: relating to or possessing a testa or hard shell;
           "testaceous fish"