Fairy Inkcap, Fairies Bonnets

Cap: Minute to 2 cm; oval when young, expanding to broadly convex or bell-shaped; when young almost white, with a brownish center--or grayish--darkening to grayish or grayish brown with a brownish center, paler towards the margin; smooth, or very finely granular/hairy when young; lined or grooved from the margin nearly to the center.

source : http://www.mushroomexpert.com/coprinellus_disseminatus.html