Agaricus bitorquis, Sidewalk Mushroom .
This mushroom is so uncommon it does not have an official English name, thus the American appellation. It is a fairly large, compact mushroom, characterised by the two rings on the stem. The cap reaches 10cm in diameter, and the stem up to 6cm, and very stout. Gills turn a deep chocolate brown with age. It is said to prefer hard-packed or disturbed soil, but the only recorded specimens were found under Cupressus trees. It may have been overlooked in this habitat. It is a good edible mushroom, and is sold in mushroom-kits