The Collaborative International Dictionary of English v.0.48:

Marasmius \Marasmius\ prop. n.
   A genus of chiefly small white-spored mushrooms, including
   the Marasmius oreades, the mushroon that grow in a fairy
   ring.

   Syn: genus Marasmius.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Marasmius
    n 1: chiefly small mushrooms with white spores [syn:
         Marasmius, genus Marasmius]