Agaricus crocodilinus

Agaricus crocodilinus was described from western North America for a giant white grassland mushroom whose cap surface breaks into broad, flattened, slightly upturned scales — a pattern that reminded its author of crocodile hide and gave the species its epithet. It is among the bulkiest Agaricus species of open ground, with caps that regularly exceed 15 cm and a stipe thick enough to make the whole fruit body genuinely heavy.

It belongs to Agaricus section Arvenses, the almond-scented group that includes the Horse Mushroom and the Macro Mushroom, and shares with them free gills that darken to chocolate brown, a substantial ring, and dark brown spores. Its taxonomic status has been debated for decades: many European authors regard it as inseparable from Agaricus urinascens, while North American workers have often kept the name for the coastal giants of California and the Pacific Northwest.

Whatever its ultimate placement, the field impression is distinctive — a great white dome sitting in short coastal turf, its surface fissured into a jigsaw of scales, appearing after the first serious rains of the season.

Cap

Very large, typically 10–25 cm across and sometimes considerably more. It starts as a compact, almost rectangular white button with an incurved margin, expands to a broad dome, and finally flattens. The surface is dry and matt, white to cream ageing to buff or pale tan, and characteristically cracks and splits into broad flat scales or angular plates, particularly toward the centre and in dry weather. The margin often bears hanging veil fragments.

Gills

Free from the stipe, extremely crowded, and narrow. They are pale grey to greyish-pink in youth — not bright pink — then pass through dull pink and pinkish-brown to a dark chocolate brown at maturity.

Stipe

Short and very thick relative to the cap, commonly 6–12 cm long and 2–5 cm wide, solid, often tapering downwards to a narrowed or bluntly pointed base. It is white and smooth above the ring, more fibrous or scurfy below. The ring is thick and pendulous, frequently double-layered, and its underside can be felty or split into patches.

Flesh and spore print

Flesh is thick, dense, and white, unchanging or developing only a slow dull pinkish to brownish tint where cut. The odour is sweet and almond-like when fresh, turning sharper in age. The spore print is dark brown, and the spores are large.

A grassland fungus of coastal and near-coastal country: cattle pasture, coastal prairie, dune grassland, meadow edges, and grassy terraces above the shore, generally on rich or lightly grazed soils. It also turns up in large mown areas such as playing fields and parkland where the turf is old and undisturbed. It typically fruits in scattered troops, arcs, or partial rings.

In western North America the main season is autumn into winter, following the onset of the rainy season, with fruiting continuing through mild winter spells. European records under this name are chiefly summer and autumn. The species is best known from the Pacific coast of North America, with reports from Mediterranean and Atlantic Europe under the same or closely allied names.

This is a saprotrophic grassland fungus. The mycelium forms a dense mat in the thatch and upper soil, digesting dead grass roots, leaf bases, and accumulated organic debris, and in grazed sites also processing dung-enriched material. The resulting release of nitrogen and phosphorus often shows as a band of darker, more vigorous grass just behind the fruiting front.

Colonies expand outward radially over many years, so productive sites tend to be old ones and the same rings can be productive for decades. Because the mycelial mat lies in the top few centimetres of soil, the species is sensitive to ploughing, reseeding, and heavy fertiliser application, and it is most reliably found where the sward has been left alone.

Agaricus urinascens, the Macro Mushroom, is so similar that many mycologists treat the two as one species; where they are separated it is on spore measurements and on the coarser, more plate-like cap cracking of A. crocodilinus. The Horse Mushroom (Agaricus arvensis) is smaller, has a silky smooth cap that stays intact rather than cracking into plates, and bruises a clear yellow on cap and stipe base.

The Field Mushroom (Agaricus campestris) is far more slender, with a thin evanescent ring and bright rose-pink young gills. Large white Amanita species growing in the same open country are separated instantly by white gills that never darken, a white spore print, and a bulbous or sac-like volva at the stipe base, none of which occurs here.