Agaricus campestris is a widely eaten gilled mushroom closely related to the cultivated button mushroom Agaricus bisporus. It is commonly known as the field mushroom or, in North America, meadow mushroom. Taxonomy This species was originally noted and named in 1753 by Carl Linnaeus as Agaricus campestris. It was placed in the genus Psalliota by Lucien Quelet in 1872. Some variants have been isolated over the years, a few of which now have species status, for example, Agaricus bernardii Quel. (1878), Agaricus bisporus (J.E. Lange) Imbach (1946), Agaricus bitorquis (Quel.) Sacc. (1887), Agaricus cappellianus Hlavacek (1987), and Agaricus silvicola (Vittad.) Peck (1872). Some were so similar they did not warrant even varietal status, while others have retained it. Agaricus campestris var. equestris (F.H.Moller) Pilat (1951) is still valid. A. campestris var. isabellinus (F.H.Moller) Pilat (1951), and A. campestris var. radicatus, are possibly still valid too. The Latin specific epithet campestris means "of the fields". Common names given to the fungus include "meadow mushroom", "pink bottom", and "field mushroom". An analysis of ribosomal DNA of a limited number of members of the genus showed A. campestris to be an early offshoot in the genus and sister taxon to A. cupreobrunneus. Description The cap is white, may have fine scales, and is in diameter; it is first hemispherical in shape before flattening out with maturity. The gills are initially pink, then red-brown and finally a dark brown, as is the spore print. The stipe is tall, 1–2 cm wide, predominantly white and bears a single thin ring. The taste is mild. The white flesh bruises a dingy reddish brown, as opposed to yellow in the poisonous Agaricus xanthodermus and similar species. The thick-walled, dark brown, elliptical spores measure 5.5–8 μm by 4–5 μm. Cheilocystidia are absent. Similar species Several species may be confused with Agaricus campestris. The most dangerous confusion may be with Amanita virosa, which is morbidly toxic, or with the deadly Amanita hygroscopica or 'Pink-Gilled Destroying Angel'. A less serious, but more common, confusion is with Agaricus xanthodermus ("the yellow stainer"), which causes gastrointestinal problems in many people. In the US, it may be confused with the poisonous Agaricus californicus or Agaricus hondensis. Agaricus arvensis, the horse mushroom, is another similar mushroom, and an excellent edible. White Clitocybe species that also grow on lawns, and in grassy places may be dangerous to eat. Distribution and habitat Agaricus campestris is found