Agaricus arvensis, commonly known as the horse mushroom, is a sometimes massive agaric that has a white cap, ringed stem, and pink or chocolate colored gills. It grows on soil (often heavily fertilized) or on vegetable debris.
white or cream colored, may have fine scales, first hemispherical in shape before flattening out with maturity. Flesh is white, firm and thick. Gills
are at first white, becoming pink, then chocolate brown or blackish, free and crowded. Stem
is white or cream; it is slightly club-shaped and smooth or finely scaly below the ring. Ring is white or cream, pendulous and superior. Viewed from below, on a closed-cap specimen, it has a well-developed 'cogwheel' pattern around the stem.
There are several similar species. Agaricus macrosporus are very fleshy, with scaly stem girdles. Agaricus sylvicola is a woodland variety. Agaricus augustus and Agaricus xanthoderma are also similar. Be careful not to confuse it with the deadly poisonous Amanita virosa, which has white gills.
on the www.first-nature.com web site.