Marasmius rotula

Scientific name:  Marasmius rotula (Scopoli:Fries) Fries
Derivation of name:  Rot- means "wheel" referring to wheel-
like appearance of the cap and widely spaced gills.
Synonyms:  
Common name(s):  Pinwheel Marasmius; Horse hair fungus;
Little wheel.
Phylum:   Basidiomycota
Order:   Agaricales
Family:   Marasmiaceae
Occurrence on wood substrate: Saprobic; in clusters on dead
deciduous wood such as twigs and moss-covered logs and
stumps; May thought October.   
Dimensions:  Caps 3-20 mm wide; stipes 1.5-8.5 cm long and
0.3-1 mm thick.   
Cap: White to yellowish; convex with a navel-like central
depression; flesh very thin.       
Gills: Whitish; attached to a collar which surrounds the stalk.
Spore print:White.
Stipe:Whitish near the apex but mostly reddish-brown to
blackish; shiny, smooth, and hairless.
Veil: Absent.
Edibility: Unknown.
Comments: As with most Marasmius species, Marasmius
rotula is able to dry down and then revive with rain, making it
seem to appear overnight.

More information at MushroomExpert.com:  


Figure 1. The pinwheel Marasmius in a mossy habitat.
Photo © William Roody.

Figure 2. The gills are attached to a collar (collarium)
which may be free from the stipe or collapsed up against
it. Photo © Dianna Smith.