Fairy Inkcap, Coprinellus disseminatus,

The Fairy Inkcap, Coprinellus disseminatus, rarely ventures forth alone or even with just a few friends; more often it forms dense masses swarming over rotting tree stumps and roots. These gregarious little fungi occur from early spring until the onset of winter, and they are at their most spectacular when the caps are young and pale - sometimes nearly pure white. It takes just two or three days for young white caps to turn grey and then begin blackening.