How to tell Horse mushroom from Garland roundhead

The cap is 6-20 cm across; in young mushrooms it's firmly hemispherical to egg-shaped, often with the margin rolled under, later becoming convex to flat. The color is pure white to cream, with subtle yellowish tints in old mushrooms; it stains yellow to ocher when touched. The surface is smooth, slightly sticky in wet weather, dry and matte in dry weather, sometimes with fine fibrous scales in the center.

The cap is small, 2 to 5 cm across and only rarely up to 8 cm. It starts out hemispherical, becomes convex and finally almost flat, and in old specimens the margin may turn upwards.

The colour is lemon yellow, golden or ochre, a little darker in the centre, and it fades to pale yellow with age and sun. The surface is smooth and slightly sticky or greasy in wet weather, but it dries quickly, and old or dried-out caps can become finely scaly. Young caps often have white scraps of veil hanging from the margin.

The gills are free and crowded, grayish-white with a pinkish tinge when young, becoming dark brown to chocolate-blackish with age. The spore print is dark brown.

The gills are attached to the stem and crowded, with many shorter gills in between. They begin pale lilac-grey, turn purplish grey and end up dark purple-brown to almost black. The gill edges are paler and look slightly fluffy. The gills are the quickest way to rule out an Agaricus: field mushrooms have free gills that are pink when young.

The spore print is dark purple-brown to blackish. The spores are smooth and ellipsoid, roughly 7–10 × 4–5.5 µm.

The stem is 6-12 cm tall and 1.5-3 cm thick, stout, fleshy and cylindrical, slightly thickened at the base, and white to cream. Near the top is a large membranous double ring with a characteristic "toothed" lower edge (like a cogwheel), a key feature of the species.

The stem is short and sturdy, 2 to 6 cm tall and 4 to 10 mm thick. It is white, sometimes yellowing with age, smooth or faintly fibrous, solid at first and later hollow. White strands of mycelium are often visible at the base.

The ring is the key feature. It is membranous, white and sits on the upper half of the stem, and it stays put as the mushroom ages. Its upper side is finely grooved, and as the cap matures the falling spores settle on it and darken it to purple-brown. There is no volva at the base.

The flesh is firm, meaty and white; when cut, it slowly yellows in the cap and upper stem. It does not redden (unlike A. silvaticus). The smell is distinctly anise-almond, a characteristic feature of the species.

The flesh is white and firm, thick in the centre of the cap and thinner towards the edge. It doesn't change colour when cut or broken. The taste is mild, not bitter.

Distinctly anise-almond and fruity, and this is the most telling feature. Rub the flesh between your fingers to bring it out. It's one of the most pleasant smells you'll come across while foraging.

The smell is faint and slightly radishy, and sometimes barely there at all. Field guides don't describe a mealy smell for this species: that belongs to its dung-loving relative, the dung roundhead. So smell won't identify it; check the gills and the ring instead.