-- field key --

2 Lepiotoid:

Cap and stem fleshy.
Gills free from stem, and usually conspicuously so. 

Partial veil present, leaving a membranous ring of tissue on stem.
[The salmon-spored Pluteus is similar but lacks a partial veil.]

Lepiota procera
with S. Siegler

Smooth Lepiota
Lepiota naucina
Onion-stalked Lepiota
Lepiota cepaestipes

Reddening Lepiota
Lepiota americana

Green-spored Lepiota
Chlorophyllum molybdites -- mature
immature specimen --
the gills and spores get green later when mature

Look alikes include Agaricus and, also, the pink to salmon-spored Pluteus (without a partial veil).

Meadow mushroom
Agaricus campestris
Agaricus sp.
section Xanthodermus complex

Fawn mushroom
Pluteus cervinus
Pluteus cervinus v. alba