Agaricus is a genus of mushrooms that includes common edible varieties like button, portobello, and field mushrooms, as well as poisonous species. Key characteristics of the genus include free-from-the-stem gills that darken with age, a ring on the stem from a partial veil, a lack of an "egg sack" at the base, and a spore print that is dark brown. A crucial test for distinguishing many poisonous species is if the flesh or cap turns yellow when bruised, indicating a "yellow stainer".