1) Aquatic fern 2) Regnellidium diphyllum 3) Water fern 
Regnellidium is a monotypic genus of ferns of family Marsileaceae. The single living species, Regnellidium diphyllum, Two-leaf Water Fern, is native to South Eastern Brazil and adjacent regions of Argentina. It resembles its relatives from the genus Marsilea, but has 2-lobed leaves (rather than 4). This fern is sometimes grown in aquaria. It is th...
(from the article `Marsileaceae`) ...has cloverlike leaves with four leaflets and is widely distributed. Pilularia (pillwort), also nearly cosmopolitan, with six species, has ... ...(nutlike receptacles) along petiole; heterosporous.Family MarsileaceaeThree genera—Marsilea (waterclover), Pilularia, Regnellidium—with about 75 ....
[n] - small latex-containing aquatic fern of southern Brazil
Regnellidium diphyllum noun small latex-containing aquatic fern of southern Brazil