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]WordNet (r) 3.0 (2006):
hymenium n 1: spore-bearing layer of cells in certain fungi containing asci or basidia