*proostracum*

‖n.; pl. Proostraca [ NL., fr. Gr. &unr_; before + &unr_; 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 ]