The fruit body is a mushroom with a cap atop a central stem. Caps come in various colours, are convex to flat; smooth, fibrillose or scaly; dry; from a centimetre or so to well over 10 centimetres in diameter. Given the size variation in the cap it is not surprising that the stems also vary greatly in size. White is the most common stem colour.
The gills are initially pale pinkish and appear to change colour to dark brown with age, but it is not the gill tissue that is changing colour. Spores are very pale when immature but become dark brown at maturity and it is the mass of mature spores that gives that colour. In some species spores mature early, in others late. Hence, in one species you may see a small, open cap with dark brown gills, but in another species a much larger, open cap but where the gills are still pink.
Technically the gills are described as free. This means that none of the gills reach the stem. What you see immediately around the top of the stem is a narrow circular furrow, as in this photo: http://www.cpbr.gov.au/fungi/images-captions/agaricus-sp-0037.html.
Very few genera have free gills so it is a very informative feature.
Warning: When a mushroom starts drying, the flesh distorts and in those species without free gills, the gills may tear away from the stem and appear free. Always be suspicious when you think you’ve seen free gills. Check carefully for signs of tearing, especially if the cap shows any signs of the wrinkling that comes with drying.
There is a partial veil that covers the gills in an immature mushroom. As the cap expands this breaks at the cap margin and leaves a membranous (and often skirt-like) ring of tissue around the stem There is no universal veil. In this photo (http://www.cpbr.gov.au/fungi/images-captions/agaricus-broken-veil-0036.html) you see a skirt-like veil remnant. The earlier photo shows a smaller ring.
Spore print: chocolate brown.
The mushrooms appear on the ground in a wide variety of habitats.
Some species of Agrocybe have partial veils and brown gills – but the gills are not free and the somewhat Agaricus-looking ones are most likely to appear on wood (live or dead).
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Agaricus sp. has been recorded at: