Definition for Marasmius oreades
From Biology Forums Dictionary
This mushroom is often called the fairy ring mushroom, since it frequently fruits in rings on lawns, in meadows, and in other grassy places. Many other mushrooms create fairy rings, however, and if the e-mails I often receive are any indication, the use of the common name "fairy ring mushroom" for Marasmius oreades leads to substantial confusion, causing Chlorophyllum molybdites, Agaricus campestris, and other mushrooms to be mislabeled.