The Collaborative International Dictionary of English v.0.48:

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]