The Collaborative International Dictionary
Proostracum
Proostracum \Pro*["o]s"tra*cum\, n.; pl. Pro["o]straca. [NL., fr. Gr. ? before + ? shell of a testacean.] (Zo["o]l.) The anterior prolongation of the guard of the phragmocone of belemnites and allied fossil cephalopods, whether horny or calcareous. See Illust. of Phragmocone.
Wiktionary
proostracum
n. (context zoology English) The anterior prolongation of the guard of the phragmocone of belemnites and allied fossil cephalopods, whether horny or calcareous.