This is the mushroom grown for commercial use and what you usually buy from grocery stores. It is characterized by having two spored basidia. Examine specimens of various ages. Mount a small portion of a gill in water and look for basidia under the microscope. This is illustrated in the attached figure.
Spores are oval to round, smooth and thin-walled and the spore prints are white to cream colored.
An inner veil is also typically present. This family also includes the most poisonous mushrooms and accounts for most deaths due to mushroom poisonings in the United States. Amanita - In this genus a volva is typically present. Examine available specimens. These species are pure white and have a sac-like volva, white annulus and smooth cap.