The Collaborative International Dictionary
Nemathecium
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 alg[ae], consisting of an external mass of filaments at length separating into tetraspores.
Wiktionary
nemathecium
n. (context botany English) A kind of fructification on certain red algae, consisting of an external mass of filaments that separate into tetraspores.