periostracum(redirected from periostraca)
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periostracumthe outermost of the three layers of the shell of molluscs or brachiopods, composed of CONCHIOLIN. On the inner side lie the prismatic layer and the nacreous layer.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005