Regnellidium

{{Short description|Genus of ferns}}

{{Italic title}}

{{Automatic taxobox

| image = Regnellidium diphyllum.jpg

| taxon = Regnellidium

| authority = Lindm.

| type_species = Regnellidium diphyllum

| type_species_authority = Lindm.

| subdivision_ranks = Species

| subdivision =

• R. diphyllum Lindm.
• †R. upatoiensis Lupiaet al.

}}

Regnellidium is a monotypic genus of ferns of family Marsileaceae.{{cite web |title=Regnellidium Lindm. {{!}} Plants of the World Online {{!}} Kew Science |url=https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:295537-2 |website=Plants of the World Online |access-date=3 November 2021 |language=en}}

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 (rather than 4). 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.{{Cite journal | doi = 10.1086/317567 | last1 = Lupia | first1 = R. | last2 = Schneider | first2 = H. | last3 = Moeser | first3 = G. M. | last4 = Pryer | first4 = K. M. | last5 = Crane | first5 = P. R. | year = 2000 | title = Marsileaceae Sporocarps and Spores from the Late Cretaceous of Georgia, U.S.A | journal = International Journal of Plant Sciences | volume = 161 | issue = 6| pages = 975–988 | s2cid = 16981770 }}

The genus name of Regnellidium is in honour of Anders Fredrik Regnell (1807–1884), a Swedish physician and botanist.{{cite book | last=Burkhardt | first=Lotte | title=Verzeichnis eponymischer Pflanzennamen – Erweiterte Edition |trans-title=Index of Eponymic Plant Names – Extended Edition | publisher=Botanic Garden and Botanical Museum, Freie Universität Berlin | year=2018 | isbn=978-3-946292-26-5 | url=https://doi.org/10.3372/epolist2018 |format=pdf |language=German |location=Berlin | doi=10.3372/epolist2018 |access-date=1 January 2021}}

It was first described and published in Ark. Bot. Vol.3 (Issue 6) on page 2 in 1904.