nProöstracum(Zoöl) The anterior prolongation of the guard of the phragmocone of belemnites and allied fossil cephalopods, whether horny or calcareous. See Illust. of Phragmocone.
Century Dictionary and Cyclopedia
nproostracumThe broad and projecting lamella of the thick covering of the phragmacone of a cephalopod, extending beyond the base of the phragmacone, and being a continuation of the wall of the most anterior chamber of the shell; the foremost part of the guard or rostrum of a fossil cephalopod of the belemnite group. It is variously shaped, usually lamellate, and with the rostrum represents the pen of the squids. See cuts under belemnite, Belemnitidæ, and calamary.
Chambers's Twentieth Century Dictionary
nProostracumprō-os′tra-kum the forward continuation of the guard or rostrum in the Belemnites.