Aptychus

‖n. [ NL., fr. Gr. 'a priv. + &unr_;, &unr_;, fold. ] (Zool.) A shelly plate found in the terminal chambers of ammonite shells. Some authors consider them to be jaws; others, opercula. [ 1913 Webster ]

‖n.; pl. Anaptichi [ NL., fr. Gr. &unr_; unfolding; 'ana` back + &unr_; to fold. ] (Paleon.) One of a pair of shelly plates found in some cephalopods, as the ammonites. [ 1913 Webster ]