Source : Webster's Revised Unabridged Dictionary (1913)

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 [ae]stivation.