The Collaborative International Dictionary of English v.0.48:

Epiphragm \Ep"i*phragm\, n. [Gr. ? a covering, lid, fr. ? to
   block up.] (Zo["o]l.)
   A membranaceous or calcareous septum with which some mollusks
   close the aperture of the shell during the time of
   hibernation, or estivation.
   [1913 Webster]