Agaricus bisporus
https://www.mushroomexpert.com/Agaricus_bisporus.html
Ecology: Scattered on pizzas, gregarious on salads, densely clustered in grocery stores—and occasionally scattered to gregarious on manured soil, compost piles, in lawns, and so on, as a native species and as an escapee from cultivation (see discussion above), especially in coastal California. The illustrated and described collections were purchased in Illinois grocery stores.
Cap: 2.5-14 cm, convex to nearly round at first, expanding to broadly convex or nearly flat; dry; bald or with pressed-down fibers or very small scales; white in some cultivated varieties, brown in others; often bruising pinkish to reddish when rubbed.
Gills: Free from the stem; close; short-gills frequent; pinkish to pink at first, becoming dark brown to nearly black.
Stem: 2-7 cm long; 1-2.5 cm thick; more or less equal; bald or with small scales; white, often bruising reddish to brownish; with a thin ring that sometimes disappears in maturity.
Flesh: White and firm; usually bruising and staining pinkish to reddish, then slowly brownish.
Odor and Taste: Pleasant.
Chemical Reactions: KOH negative on cap surface.
Spore Print: Dark brown—but sometimes difficult to obtain from store-bought mushrooms.
Microscopic Features: Spores 6-8 x 5-6 m; ellipsoid; smooth; thick-walled; brownish in KOH. Basidia mostly 2-spored. Cheilocystidia 20-30 m long; clavate; smooth; thin-walled; hyaline in KOH. Pleurocystidia not found.
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