Source : Webster's Revised Unabridged Dictionary (1913)

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