Agaricus micromegethus

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)

Gills free
Spore print chocolate brown
Annulus almost always present, usually membranous
The gills are usually pink or silvery-grey at first, but are colored chocolate brown at maturity from the developing spores
The cap and stalk are usually some sort of white or greyish brown, but may have fibrils or scales that are darker (like the portobello)
Growing on the ground, wood chips, or other organic debris

Cap up to 2" across
Flesh stains or ages yellow, and stays yellow (on its own, and with KOH)
Partial veil has only one layer; typically it forms a very fragile ring, often disappearing in age
Odor sweet, sometimes faint

Cap fibrilose
Stalk staining yellow, orange or rusty orange below the ring

Agaricus micromegethus     Peck