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.

2. (Bot. & Zo["o]l.) Having a dull red brick color or a brownish yellow color.

Testaceous animals (Zo["o]l.), 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.

Source: Webster's Revised Unabridged Dictionary (1913)