Psilocybe cubensis a.k.a. Stropharia Cubensis
P. cubensis is the most frequently bought, sold, and cultivated psychoactive mushroom in the world, containing the drugs psilocybin and psilocin. It can be found growing wild in pastures in virtually any temperate or tropical part of the earth.
One of the largest psilocybin mushrooms, P. cubensis can often be 15 cm tall and 7 cm in cap diameter. It has a white stalk and a golden cap. The spores of P. cubensis are dark purple-brown. The cap starts off dark, but becomes lighter with age. Like most psychoactive mushrooms, P. cubensis will bruise blue, due to the breakdown of psilocin.
NOTE: This writeup is simply to provide a definition for the purpose of collecting knowledge on E2. I have NOT provided sufficient information here to help you hunt for P. cubensis in the wild. For that, you will need an identification guide such as Psilocybin Mushrooms of the World (by Paul Stamets, Ten Speed Press, 1996), as well as the help of an experienced mushroom-hunter. There are poisonous species that could be mistaken for P. cubensis.