Gymnopus dryophilus  (Bull.) Murrill (1916)
Syn: Collybia dryophila

Collection: Bulgaria, under Quercus cerris

This is one of the Cosmopolitan species with distribution pretty much spanning the Northern Hemisphere. I have collected it across North America and Europe where Oak is present. In Southern Europe it sometimes is the only species fruiting during the scorching summer streaks (the photo above). It is a popular edible mushroom in Eastern Europe. The cap color seems to vary with the humidity of the air. In drier weather it tends to pale out.  There is a peculiar variety from the Eastern USA, which doesn't quite looks like it, but the name Gymnopus dryophilus has been applied to it.

Collections: California, under Quercus agrifolia

Collections: California, under Quercus kelloggii and Quercus garryana

Sierra Nevada, California, under Black Oak

Collection: Bulgaria, under Quercus cerris

Collections: North Eastern USA -- Connecticut & New York State in mixed forest of White Oak and Eastern White Pine. Superficially, these collections seem much more closely aligned with Rhodocollybia butyracea, but an examination of the cuticle shows the typical dryophila structures. These probably deserve a variety status. The collections illustrated on seem to depict that group quite well.