The giant clitocybe has a huge white to buff cap 4-18 inches across, first convex, then flat, finally funnel-shaped.
Giant Clitocybe Cap
Note the flattened shape in middle age.
The margin begins inrolled and becomes furrowed. I find the odor mildly fishy.
The broad white to buff gills, sometimes forked or branched, and crowded together, descend the stalk. The spore print is white.
Giant Clitocybe, from below
Note the crowded gills that slightly descend the stalk.
The dry, solid, smooth, whitish stalk is 1-4 inches long, 1-2 inches thick.
The mushroom grows in open woods, on disturbed soil, and in gardens, from the middle of the summer through fall, throughout the US.
Giant Clitocybes in the Leaf Litter
The large white caps of these clustered mushrooms, along with their fishy smell, makes them quite distinct.
Although this large mushroom is easy to identify (there just aren't very many mushrooms that get so large, especially white ones with gills descending the stem), it has no flavor, and isn't worth collecting.