These fungi grow in fields and meadows, usually in rings and particularly where horses are usually kept (hence the common name, the Latin name arvensis means "of the field"). Unlike true Field mushrooms (Agaricus campestre), the upper surface of the cap smells strongly of aniseed. Horse mushrooms are just as delicious as field mushrooms though and are perfect with eggs and bacon.
Key identifcation points:
Cap: 3-10cm, very rounded when young flattening out as they grow, slightly scaly, bruising slightly yellow Stem: white, broader at the base
Scent: strong aniseed scent
Gills: white to pink and then to brown as the mushroom ages
Spore print: dark brown
Habitat: woodlands, pasture, parks and gardens
Season: summer to late autumn
Yellow Stainer (Agaricus xanthodermus)
Superficially very similar to both Field Mushrooms and Horse mushrooms but varies in a few very obvious ways. Yellow Stainer bruises a bright neon yellow particularly round the edge of the cap and at the base of the stem. It also has a very unpleasant chemical smell similar to TCP or carbolic soap. If you're not sure then pop a small piece into the microwave and cook on full power for 1 minute, if it's Yellow Stainer the chemical smell will be very obvious.
Yellow Stainer is poisonous and will give anyone who eats it a very upset stomach similar to food poisoning.