Can anyone help ID this please?

Can anyone help ID this please?

Observation - Can anyone help ID this please?. Description: Found in leaf litter beneath what I'm fairly sure is a lime (see leaf sample). starts out convex but then flattens, distinctive brown rim to cap, gills are distant, stem, cap and gills all pure white, stem slightly fibrillose

Found in leaf litter beneath what I'm fairly sure is a lime (see leaf sample). starts out convex but then flattens, distinctive brown rim to cap, gills are distant, stem, cap and gills all pure white, stem slightly fibrillose