aptychus

from The Century Dictionary.

• noun In Cephalopoda, a plate formed of a shelly substance, found in the terminal chamber of certain fossil mollusks, as ammonites, and regarded by some as an operculum.

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

• noun (Zoöl.) A shelly plate found in the terminal chambers of ammonite shells. Some authors consider them to be jaws; others, opercula.

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

• noun A marine fossil consisting of a hard anatomical structure like a curved shelly plate, once part of the body of an ammonite.