Pleurocybella porrigens

This is a key to gilled mushrooms, that is, mushrooms having a definite cap with a fertile surface consisting of gills. The fruiting body usually also has a stem, although that may be lateral or absent (usually, then, the mushroom is growing from wood). You can use this key to identify mushrooms that you find.

Fruiting body containing fibers (usually in the stalk)

Spore print "light-colored": white or buff, sometimes tinged with pink or tan. Greenish and (except for the Russulales) yellow spore prints also go here
Stalk fibrous, not fracturing like a piece of chalk

None of the special features distinguishing the other white-spored genera:
Gills not free, as in the Lepiotas and Amanitas
Basidia not extra-long, as in the Hygrophoraceae
Spores smooth, except for Lentinellus

Growing on trees or dead wood, leaves, or sticks, or organic debris, often in moss

Assume a lateral stem, unless otherwise noted in the diagnoses below
If laterally stemmed, then gills strongly decurrent

Cap always smooth (perhaps thinly, minutely furry), never scaly or thickly or coarsely hairy
Gills white to buff, though perhaps discoloring yellow (along with cap) in age, unless covered by a veil
Growing on wood

Cap always white
Flesh thin, not exactly fragile, but very easy to pull apart, never yellowing in age like Pleurotus species
Spores always less than 8 µm the long way, inamyloid, acyanophilic

Pleurocybella porrigens     (Persoon: Fries) Singer