Collection location: Mar Vista, Los Angeles, California, USA [Click for map]
This mushroom often gets called Agaricus arvensis in the Los Angeles area. It has the typical almond scent and the yellow staining. However, its stature is much more like A. campestris. Like that species it has a slight textured (fibrillose) pileus rather than the smooth pileus characteristic of A. arvensis. It also has a nearly solid stipe, again more like A. campestris than A. arvensis. It also has a sterile margin on the cap.
A collection of these mushrooms was submitted to the herbarium at San Francisco State University.
Observation 4220 may be the same species.
Here are some more detailed notes that I dug up on this collection:
Pileus 30-60mm broad. at first convex, with inrolled margin, later broadly convex; surface dry, appressed-fibrillose, most specimens white to off-white, a few with medium brown patches, surface staining yellow and remaining yellow; context white unchanging, firm, up to 10mm thick; taste and odor sweet and almondy; seems to have a thin sterile margin, definitely with appendiculate velar material.
Lamellae free, close, up to 9mm broad, at first pink, fading to grey pink after collecting, dark blackish-brown when mature; margin not distinctive.
Stipe 30-60mm x 10-20mm with the thickest often the shortest and youngest, thining and elongating with age; tapering downward when young, becoming equal with age; interior white, solid when young, developing a thin stuffed or hollow area in age; in the most extreme case the stipe was 14mm thick and the hollow area was only 3mm of that; surface appressed-fibrillose, breaking up into small patches as the stipe elongates; surface white unchanging.
Veils thin, intermediate to sub-pendant, superior, often disappearing from stipe in age; in all cases leaving appendiculate remnants on the pileus margin. Upper surface finely striate; margin fimbrate; lower surface fibrillose to floccose.
Location – growing in a park lawn in Los Angeles county.