The Collaborative International Dictionary of English v.0.48:

Nemathecium \Nem`a*the"ci*um\ (? or ?), n.; pl. Nemathecia.
   [NL., fr. Gr. nh^ma a thread + ? a box.] (Bot.)
   A peculiar kind of fructification on certain red algae,
   consisting of an external mass of filaments at length
   separating into tetraspores.
   [1913 Webster] Nemathelminthes