Pleurotus ostreatus

Common Name: Oyster Mushroom
Description: Cap 6-14 cm across, shell-shaped, convex at first, then flattening or slightly depressed; often wavy and lobed at the margin or splitting; rather variable in color, but deep bluish gray to gray-brown when young, buff-brown with age.
Habitat: in large clusters on stumps, logs, and trunks of deciduous trees. Common. Found throughout North America.
Edible: delicious.

Source: Mushrooms of North America by Roger Phillips.