The Collaborative International Dictionary of English v.0.48:

Lithocyst \Lith"o*cyst\ (l[i^]th"[-o]*s[i^]st), n. [Litho- +
   cyst.] (Zool.)
   A sac containing small, calcareous concretions (otoliths).
   They are found in many Medus[ae], and other invertebrates,
   and are supposed to be auditory organs.
   [1913 Webster]