Marasmius undatus) is a small mushroom which grows on fern rhizomes.

[5] This species was originally described in 1836 as Agaricus undatus by Miles Joseph Berkeley.

As it happened, in 1838 the famous mycologist Elias Magnus Fries defined a completely different mushroom (now Entoloma undatum) under the same name, but due to the nomenclatural rules of precedence, that definition is illegitimate.

This was confirmed when in 1849 Fries renamed Berkeley's fungus with the combination Marasmius undatus, a name which was current for more than a century.

[1][6] This mushroom grows on rhizomes of bracken (Pteridium aquilinum) and probably other ferns.