The Collaborative International Dictionary of English v.0.48:

Hymenium \Hy*me"ni*um\, n.; pl. L. Hymenia, E. Hymeniums.
   [NL., fr. Gr. ? a membrane.] (Bot.)
   The spore-bearing surface of certain fungi, as that on the
   gills of a mushroom.
   [1913 Webster]