Agaricus is a distinctive genus, with free gills and dark chocolate spores usually being enough to identify it. They always have a partial veil that leaves at least some trace of a ring around the stem.
They have dry caps, often fibrillose or scaly. The spores will sometimes turn to pink and then dark chocolate, so you may find young specimens with pink gills. But the spores will never drop from the gills until they are mature and dark chocolate, so if you take a spore print, you will never be fooled. You'll often see a robust, stocky stature. Most of them are >5cm across (with some up to 20-30cm). A few small species are <5cm, but they are still stocky with a thicker stem than most other mushrooms of that size (5mm or more thick). They are all saprophytic.
Agaricus flesh may stain yellow or reddish when cut. Some have a pleasant sweet almond odor (like almond
latte syrup or black licorice). It is very
interesting to eat a mushroom with a sweet component to it (I like them on
pineapple pizza where there is already a sweet/
Some of Agaricus' close relatives are Lepiota (which have lost their spore pigment and have white spores), the Coprinus inky cap and a surprising array of non-gilled mushrooms like puffballs.
Key to Agaricus sections: crush the stem base to detect all odors. Cut in half if you still don't see a staining reaction.
Arvenses - these species stain slightly yellowish (weakly to moderately but often strengthening over time) when you cut them in half. They have a pleasant sweet almond odor, best detected by crushing and smelling the base of the stem. KOH yellow.
Highly pigmented, with lots of warm brown fibrils or scales
Species found in grass (they will be mostly white capped).
A. fissuratus - "horse" mushroom - found in grass. White. Thick veil. Slight yellow staining. Compare A. 'campestris'. ~10cm.
A. arvensis (nivescens/
Next, forest species with relatively strong yellow staining (easier to detect).
A. albolutescens - in coastal forests. Whitish. Squat, strong yellowing and almond scent. Up to 15cm.
Lastly, forest species with subtler yellow staining.
Minores - related to the Arvenses, these are our smallest Agaricus. Often <5cm, but still somewhat stocky. Also yellowing (or turning orange, the colour often strengthening over time) with an almond scent. They are difficult to tell apart. KOH yellow.
A. kerriganii (semotus) - in forests, turns yellow-orange. Up to 7cm (largest small species) may have a bulb and rhizomorphs.
A. diminutivus group - a half dozen little known slender species found in forests, also staining yellow to orange.
A. 'purpurellus' - some A. diminutivus grp members can sometimes be a beautiful purple . For now, they're probably incorrectly being called A. 'purpurellus'.
Agaricus - normal odor, no staining reaction (rarely slightly orange). The stem often tapers and the veil leaves tissue hanging off the edge of the cap but not much of a ring compared to the other subgenera. Usually found in grass. KOH negative.
A. 'aristocratus' - white cap and lower stem fibrillose instead of smooth.
Subrutilescentes - No staining reaction. Normal mushroom odor.Well developed ring. Found deep in forests under conifers with dark vinaceous brown scaly caps. KOH green.
Xanthodermati - this is the section of "poisonous portobellos". They should smell of creosote (phenol, bad science lab) when you crush the stem base. May stain yellow (possibly very subtly and the colour usually fading over time to brown). KOH yellow.
Bivelares - slow red staining (if at all). Normal mushroom odor. The ring sometimes sheathes upwards and then outwards instead of down and out, but is always well developed (unlike the similar subgenus /Agaricus) and they usually grow in compost or disturbed areas, not deep in forests (unlike the similar /Subrutilescentes). KOH negative.
Agaricus bisporus, the store bought button mushroom, is in the /Bivelares. The portobello is also the same species, it is just bred to be larger. Check them out at the grocery store next time, and you may actually see the shape of the veil and a slight red staining before you get kicked out of the produce section, but these features are subtle.
A. bitorquis - white cap, very squat, tapering stem, thick sheathing veil, in hard packed, disturbed soil. Little red staining. 5-20cm or so.
Chitonioides - A quick or strong red staining reaction, stem almost as thick as the cap is wide and very short (very squat mushroom) that grows in salty areas and smells briny itself. Inrolled cap margin. KOH negative.
Sanguinolenti - these red stainers stain more strongly then the Bivelares/Chitoniodes (except for the squat, briny A. bernardii) and are not usually as stocky. None are very common in the Pacific Northwest, so unfortunately not a lot is known about the species to be found here. KOH-.
A. thujae - brown scaly capped group. Slender with veil remnants on cap margin and lower stem scales. With red cedar.
A. sylvaticus subsp occidentalis - small (<5 cm) pale cap (white to tan) and scaly stem. Stains pinkish-red or orangish-red.
Rick Kerrigan's new book, Agaricus of North America is a great comprehensive treaty on all that is understood so far about this genus in North America.