Pileus: convex becoming planar, dry or with scales
Annulus: usually present
Stipe: easily separates from the pileus.
Stature: Lepiotoid, Pluteoid if the annulus has disappeared
Ecology: saprophyte, in grass, woodlands, soil
: dark brown, with a germ pore (Singer
, 1986) or with or without a germ pore (Largent and Baroni
, 1988), dextrinoid to inamyloid, smooth or slightly roughened.
Hyphae: inamyloid, with, or usually without clamp connections.
Cystidia: usually none, Cheilocystidia present in some species
Spore Print: chocolate to purplish-brown
Lamellae: close to crowded, free
: regular becoming irregular (Singer
Pileipellis: usually filamentous
According to Arora, 1986, this genus is a "cinch to recognize" because it has a stalk with an annulus, no volva, gills pinkish when young, becoming chocolate brown, spore print always chocolate (or purplish) brown, and the stem separates cleanly from the cap.
If worse comes to worse, a visit to the local supermarket for the common button mushroom (Agaricus bisporus = A. hortensis?)) will always yield results. The Cremini and the Portabella are just horticultural varieties of A. bisporus. A mushroom farmer in Kennet Square, Pennsylvania told me that they used to throw away (or eat themselves) button mushrooms that "expanded". Some smart marketing agent came up with these Italianesque names, and low and behold, these new mushrooms hit the market. Now, were even seeing "baby bellas"!
Over 100 species occur in North America (Bessette et al., 1997). A campestris, A arvensis and A, bitorquis are the most common species in the Berkshires. The former two can often be collected in great numbers in the early fall/late summer.