Without any info about the underside I cannot be sure of genus. Very little of the gill area shows and I'm not sure whether the gills are dark coloured or if I'm seeing a shadowing effect. There seems to be a white remnant of a partial veil around the stem and that, combined with dark coloured gills would make this a species of Agaricus. However, it may be that what I think to be a ring of veil remnant is not so.