proostracum

from The Century Dictionary.

• noun The 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The anterior prolongation of the guard of the phragmocone of belemnites and allied fossil cephalopods, whether horny or calcareous. See Illust. of phragmocone.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology The anterior prolongation of the guard of the phragmocone of belemnites and allied fossil cephalopods, whether horny or calcareous.