Regnellidium

Regnellidium is a monotypic genus of ferns of family Marsileaceae.
The single living species, Regnellidium diphyllum, the two-leaf water fern, is native to southeastern Brazil and adjacent regions of Argentina. It resembles its relatives from the genus Marsilea, but has 2-lobed leaves. This fern is sometimes grown in aquaria.
A fossil assigned to the species Regnellidium upatoiensis has been found in Cretaceous deposits of the eastern United States.
The genus name of Regnellidium is in honour of Anders Fredrik Regnell, a Swedish physician and botanist.
It was first described and published in Ark. Bot. Vol.3 on page 2 in 1904.