Hippuris tetraphylla; The Four-leaved Mare's-tail is a perennial herb with ascending stems up to 40 cm tall. The stems grow from rootstocks.
The stems are usually red and the leaves are rather small (broadly-) oblong shaped. Typical for this species is that the leaves are arranged in whorls of four.
The flowers are minute placed in the axil's of the leaves. The filament on the stamen is much shorter than that of the more common Mare's-tail.
The species is confined to brackish water along coastal marsh-lands in a few localities of the north of Iceland where they usually grow in shallow water.
Traditionally it was a member of the Hippuridaceae family but is now regarded as to be a member of the Plantain-family (Plantaginaceae). The Icelandic name of this species is Flæðalófótur.
Next to H. vulgaris (Mare's-tail) and H. tetraphylla (Four-leaved Mare's-tail) an intermediate is also found on Iceland, being Hippuris lanceolata (Six-leaved Mare's-tail). These plants have leaves in whorls of 6 and their shape are intermediate between vulgaris and tetraphylla. It grows both in fresh water as well as in more saline water. The flowers resemble those of H. tetraphylla. Right now (2022) it is not known if this is a hybrid or a true species. The Icelandic name for this taxon is Lensulófótur.