The
pileus or cap of the original wild species is a pale grey-brown, with broad, flat scales on a paler background and fading toward the margins. It is first hemispherical before flattening out with maturity, typically measuring in diameter. The narrow, crowded
gills are free and initially pink, then red-brown, and finally a dark brown with a whitish edge from the
cheilocystidia. The cylindrical
stipe is up to tall by wide and bears a thick and narrow
ring, which may be streaked on the upper side. The firm flesh is white, although it stains a pale pinkish-red on bruising. The
spore print is dark brown. The spores are oval to round and measure approximately 4.5–5.5
μm × 5–7.5 μm, and the
basidia usually two-spored, although two four-spored varieties have been described from the
Mojave Desert and the
Mediterranean, with predominantly
heterothallic and
homothallic lifestyles, respectively.
Similar species The common mushroom could be confused with young specimens of a group of
lethal mushrooms in the
Amanita genus referred to as
destroying angels, but the latter may be distinguished by their
volva or cup at the base of the mushroom and pure white gills (as opposed to pinkish or brown of
A. bisporus). A more common and less dangerous mistake is to confuse
Agaricus bisporus with
A. xanthodermus, an inedible mushroom found worldwide in grassy areas.
A. xanthodermus has an odor reminiscent of
phenol; its flesh turns yellow when bruised. This fungus causes
nausea and vomiting in some people. The poisonous European species
Entoloma sinuatum has a passing resemblance but has yellowish gills, turning pink, and lacks a ring. ==Taxonomy==