aptychus

from The Collaborative International Dictionary of English v.0.48

Aptychus \Ap"ty*chus\, n. [NL., fr. Gr. 'a priv. + ?, ?, fold.]
   (Zool.)
   A shelly plate found in the terminal chambers of ammonite
   shells. Some authors consider them to be jaws; others,
   opercula.
   [1913 Webster]