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The horse mushroom, Agaricus arvensis, is a mushroom of the genus Agaricus. It tends to be the largest white British Agaricus species. It is found most frequently in the vicinity of stables and byres. The cap is similar to that of Agaricus campestris, but the gills are white at first (when this fungus is most often confused with deadly Amanita species), but later pass through grey and brown to become dull chocolate. There is a large spreading ring, white above but sometimes with yellowish scales underneath.
