Coprinus comatus

Common Name: Shaggy Mane
Description: Cap 3-7 cm across when expanded, more or less a tall ovoid when young, becoming more cylindrical as it expands; white and very shaggy-scaly, often with a pale brownish "skullcap" at apex. Stem 60-120 by 10-20 mm high and wide.
Habitat: often in large numbers on roadsides, lawns, and other urban sites, especially where the soil has been disturbed. Found throughout North America.
Edible: yes, and delicious when young.

Source: Mushrooms of North America by Roger Phillips.