I admit this is not a very flattering photograph for a mushroom that many people consider to represent the essential edible mushroom, the champignon of France, the wild mushroom that most resembles our store-bought button mushroom (Agaricus bisporus.) The common field mushroom, A. campestris tastes very much like the commercially grown button mushrooms, only more concentrated and deeper in complexity. Perhaps its flavor more closely resembles the large brown variety of A. bisporus sold as portobello mushrooms in California supermarkets. The mature A. campestris has brown gills, the buttons have pink gills. It does not stain yellow nor smell phenolic like A. Californicus or A. xanthodermis, nor does it have the sweet almond smell of A. arvensis. Like A. arvensis, it has a pure white cap which may turn pale tan in age (like the one pictured above). The flesh of older specimens may discolor slightly brown or reddish brown, and the smell is mild and . . . well, mushroomy. Older specimens may have some maggots, but usually not as many as some other agarics. As with other wild mushrooms, campestris usually have more moisture and softer flesh than their commercially grown cousins, but they compensate with an exceptionally rich flavor. An impeccably delicious mushroom.
Be careful never to confuse this mushroom with the deadly Amanita ocreata, the pure white destroying angel, which has a volva at the base of the stem and pure white gills.
(A note about the photo: this mushroom typically grows in grass. The pictured specimen was growing immediately next to a suburban lawn in some leaf litter. The brown on the cap turned out to be mostly dirt, and despite its maturity the mushroom was delicious!)