Agaricus arvensis (J.C. Schaeffer: Fries)

The pleasant aroma and quick yellowing tendencies distinguish the members of the Arvenses section. The meadow habitat pinpoints this species without the need for much additional pondering. This one is a common edible, popular on both sides of the Atlantic.

California collections

Agaricus arvensis (left) vs. Agaricus campestris (right) 
Collected from the same lawn. Note the difference in (1) yellowing reaction; (2) the annulus type; (3) the shape of the stem; and most importantly (4) the odor, not easily projected through this medium...