Many Agaricus have similar charactaristics, but I keyed it out to A. xanthodermus because of the bulb at the base of the stipe and the yellow staining reaction dulling to brown after a few minutes.
patterns_of_absurdity
2016-01-00
Mon Jan 18 2016 12:48:06 GMT-0800 (PST)
United States of America, California, Alpine Rd, Portola Valley, CA, US
Growing in soil under blue oaks Pinkish tinge around edge of older mushrooms Pink gills thick and some incomplete Gills do not touch stipe Staining yellow on cap and stipe but then fading quickly to brown Clear annulus Bulb at base of stipe Smooth, convex cap, bright white with brownish tinged Phenolic odor Mild flavor