Agaricus arvensis

Agaricus arvensis, commonly known as the horse mushroom, is a mushroom of the genus "Agaricus".
Taxonomy.
It was described as "Agaricus arvensis" by Jacob Christian Schaeffer in 1762, and given numerous binomial descriptions since. Its specific name arvensis means 'of the field'.
Description.
The cap is similar to that of "Agaricus campestris" (the field mushroom). The gills are pale pink to white at first (when this fungus is most often confused with deadly genus "Amanita"). They later pass through grey and brown to become dull chocolate. There is a large spreading ring, white above but sometimes with yellowish scales underneath. Viewed from below, on a closed-cap specimen, the twin-layered ring has a well-developed 'cogwheel' pattern around the stipe. This is the lower part of the double ring. The odor is described as like anise. It belongs to a group of "Agaricus" which tend to stain yellow on bruising.
Distribution and habitat.
It is one of the largest white "Agaricus" species in Britain (where it appears during the months of July–November), West Asia (Iran) and North America.
Frequently found near stables, as well as in meadows, it may form fairy rings. The mushroom is often found growing with nettles (a plant that also likes nutrient-rich soil). It is sometimes found associated with spruce.
Conservation.
This mushroom is considered common and widespread, and is not a conservation concern.
Edibility.
This is a choice edible species which has been much prized by farmers for generations, being regarded as one of the most delicious of all edible fungi. Despite this, the fruitbodies of this and other yellow-staining "Agaricus" species often have a build-up of heavy metals, such as cadmium and copper.