halymenia

from The Century Dictionary.

• noun A genus of marine algæ belonging to the natural order Cryptonemeæ, tribe Gastrocarpeæ, characterized by the cylindraceous or compressed, gelatinous, membranaceous fronds, which are dichotomous, pinnate, or variously branched, and by the simple cortical layer formed of small oblong cells, its medullary portion being formed of large cells and internal articulated branching filaments. The species are natives of the warmer seas.