phragmacone

from The Century Dictionary.

• noun The conical, spiral, or otherwise shaped and chambered or septate internal skeleton of fossil cephalopods, contained in the anterior part of the cavity of a hollow hard structure called the guard or rostrum. It is homologous with the chambered shell of other cephalopods. See cut under belemnite.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the thin conical chambered internal shell (either straight or curved) of a belemnite