Agaricus is a large and important genus of mushrooms containing both edible and poisonous species, with possibly over 300 members worldwide. The genus includes the common ("button") mushroom (Agaricus bisporus), and the Field mushroom (Agaricus campestris) the dominant cultivated mushrooms of the West.
Members of Agaricus are characterized by having a fleshy cap or pileus, from the underside of which grow a number of radiating plates or gills on which are produced the naked spores. They are distinguished from other members of their family, Agaricaceae, by their chocolate-brown spores. Members of Agaricus also have a stem or stipe, which elevates the pileus above the object on which the mushroom grows, and a partial veil, which protects the developing gills and later forms a ring or annulus on the stalk.
Several origins of Agaricus have been proposed; It possibly derives "from Agarica of Sarmatica, a district of Russia" (!). Note also Greek ἀγαρικόν "a sort of tree fungus" (There's been an Agaricon Adans. genus, treated by Donk in Persoonia 1:180)
Donk reports Linnaeus' name is devalidated (so that the proper author citation apparently is "L. per Fr., 1821") because Agaricus was not linked to Tournefort's name (Linnaeus places both Agaricus Dill. and Amanita Dill. in synonymy), but truly a replacement for Amanita Dill., which would require that A. quercinus, not A. campestris be the type. This question compounded by the fact that Fries himself used Agaricus roughly in Linnaeus' sense (which leads to issues with Amanita), and that A. campestris was eventually excluded from Agaricus by Karsten and was apparently in Lepiota at the time Donk wrote this, commenting that a type conservation might become necessary.
The alternate name for the genus, Psalliota, derived from the Greek ''psalion/ψάλιον, "ring", was first published by Fries (1821) as trib. Psalliota. The type is Agaricus campestris (widely accepted, except by Earle, who proposed A. cretaceus). Paul Kummer (not Quélet, who merely excluded Stropharia) was the first to elevate the tribe to a genus. Psalliota was the tribe containing the type of Agaricus, so when separated, it should have caused the rest of the genus to be renamed, not what happened. It seems to be currently not considered valid, or a junior homotypic synonym, anyway the explanation is that it was raised by (in retrospect) erroneously maintaining the tribe name.
One species reported from Africa, A. aurantioviolaceus, is reportedly deadly poisonous.