Marasmius calhouniae (Calhoun's pinwheel)

Marasmius is a genus of mushroom-forming fungi in the family Marasmiaceae. It contains about 500 species of agarics, of which a few, such as Marasmius oreades, are edible. However, most members of this genus are small, unimpressive brown mushrooms. Their humble appearance contributes to their not being readily distinguishable to non-specialists, and they are therefore seldom collected by mushroom hunters. Several of the species are known to grow in the characteristic fairy ring pattern.

— Wikipedia