Observations by old_timer

Found on ground in oak duff and soil at roadside

Cap: 11 cm dia., convex to plane. The surface is smooth and moist (rain is occurring off and on). Margin appears to have a thin remnant of the partial veil.

Stem: equal, thin, long, 1.5 cm x 11 cm hollow in Apex with pithy flesh near the base. Stains deep brown to near black at apex. Recurves appear near base. Slight library-paste aroma. Taste, none. Stem central easily separated from cap.

Stem surface: Color white near base, turning gradually from light grey to near black at the cap. Partial veil present. Possibly slightly staining on the small, slightly bulbous base.

Gills: free, densely crowded, chocolate-brown. Many gills are fully radial. If ranks occur, they are difficult to see. Intermediate gills can be seen but they are relatively few.

Summary: I tend to think that Agaricus species can be sorted on a preliminary basis by stature. Someone please correct me if it is an incorrect assumption. This species is definitely long-legged. The stem is thin and as long as the cap is round.

Staining of the skin and flesh are also diagnostic, as is the aroma.

KOH is good for winnowing out species, but not everyone has it on hand. Melzer's reagent is sometimes significant, but the chloral-hydrate is not available to hobbiests.

Summary: the cap diameter, long leggedness, greyness on stem, slight ring remnants on margin, along with faint staining and aroma lead me to Agaricus californicus.

BWTFDIK. I bow to deeper knowledge.

Cap broad, >20cm dia. wet from very recent rain.
Note cap ornamentation: dark fibrils grouped against a light background.

Stem >10 cm long x < 3cm dia. NOT(short and squat.) Like recent Agaricus type specimen below.
Flesh dense to touch with considerable heft.

Lamella free,dark, and chocolate brown. .Crowded.

We are using INaturalist here to get some baseline on the rock distribution here and what to expect at various low tides. We can easily scan from the bluffs for ochre sea stars because they usually stand out well against the rocks. This species has been waxing and waning through recent years due to a wasting disease.

(1) Tide at -3.5 ft.
Datum mean sea-level.

(2) same view Next day 0950 PST.
Tide at + 2.2 ft > msl

Tide station in nearby San Simeon.

Haven't tried to key this to species yet.

I'm confident that it's in the Agaricus family.
The stature reminds me of A. Bitorquis, but it's not pushing up soil like previous collections from Colorado or Nevada. And the cap has different ornimentation and color.

Plus, the cap on this is nearly 25 cm in diameter.