Worldwide there may be up to 300 species of Agaricus, some of which are edible, while others are poisonous. Our common button mushroom, Agaricus bisporus, is in this group. So is Agaricus campestris, the field mushroom. All agaricus have a fleshy cap, radiating gills, and a chocolatey brown spore-print. They also have a stipe and a partial veil which becomes a ring on the stalk. Agaricus arvensis is called the ‘horse mushroom.’ It has a sweet smell, cog-wheeled ring. The caps will often bruise yellow if rubbed, and the flesh will sometimes turn yellowish on exposure to air--but the flesh in the base of the stem is not yellow, which helps distinguish it from other species. 030-001