First Agaricus of the season! However I’ll be buggered if I can figure out what it is. Big bulb at the bottom (no vulva), no yellow staining, no ring, slightly inrolled margin. No clue.
Update: Chocolate brown spores. Unfortunately, the print was lost before I could look at it under the microscope when the cap liquified from maggots.
From a distance it looked like a nice cluster of Pleurotus, but up and close I found they were fuzzy and tough. With white gills and yellow-white spores (P. ostreatus and friends have lilac spores), my best guess is Panus rudis.
UPDATE: These cannot be P. rudis, as they have forked gills and a bright yellow spore print. Back to the drawing board…