The Collaborative International Dictionary of English v.0.48:

Anaptychus \An*ap"ty*chus\, n.; pl. Anaptichi. [NL., fr. Gr. ?
   unfolding; 'ana` back + ? to fold.] (Paleon.)
   One of a pair of shelly plates found in some cephalopods, as
   the ammonites.
   [1913 Webster]