The Collaborative International Dictionary of English v.0.48:

Helicotrema \Hel`i*co"tre"ma\, n. [NL., fr. Gr. "e`lix,
   "e`likos, a helix + ? a hole.] (Anat.)
   The opening by which the two scal[ae] communicate at the top
   of the cochlea of the ear.
   [1913 Webster]