There are several similar species in this genus, and the spores need to be examined under the microscope to identify.  They are small and very fragile and ephemeral with a strongly plicate (pleated) and very thin cap.  There is some variation in colour, but they are generally brownish when young, becoming pale grey as the cap expands, with a brownish central disc. The gills are very free (i.e. not attached to the stipe) and shrivel rather than deliquesce or turn to ink when over-mature.  The spore print is black. 

Photograph from top down and underneath to show gills and full length of stipe. Note habitat and substrate

Grassland, woodchip, soil, dung

common

common