'Proostracum' definitions:

Definition of 'Proostracum'

From: GCIDE

Proostracum \Pro*["o]s"tra*cum\, n.; pl. Proostraca. [NL., fr. Gr. ? before + ? shell of a testacean.] (Zool.) The anterior prolongation of the guard of the phragmocone of belemnites and allied fossil cephalopods, whether horny or calcareous. See Illust. of Phragmocone. [1913 Webster]