Agaricus arvensis

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 7" across
Flesh stains or ages yellow, and stays yellow (on its own, or with KOH)
Partial veil usually has two distinct layers.
Mature fruiting body usually quite tall (stem 5-10" long), and often quite wide as well
Odor sweet, usually almondy, sometimes of anise, but different people will sometimes perceive it differently
The gills are greyish at first, not pink

Stem robust in proportion to the cap
Cap glabrous or fibrillose, but not silky (although the cap is sometimes shiny)
Scent sometimes quite strong
Usually in grass or wood chips

Agaricus arvensis     Ja.C. Schaeffer: Secretan