Agaricus crocodilinus Identification Guide

The First Impression

Agaricus crocodilinus announces itself by scale. You are usually walking open ground, coastal grassland, a dune slack, an old pasture or a fog-belt prairie, and a white dome the size of a fist or a dinner plate is sitting in short turf, sometimes with several others strung out along an arc. Even before you reach it the surface texture is the giveaway: the white cap skin has split into large, flat, angular plates with narrow buff-tinted cracks between them, the pattern that earns the species its reptilian name.

Get down to ground level before you touch it. Note whether the fruit bodies form part of a ring or arc, whether the turf around them is unusually dark and lush, and what the substrate is. This is an open-country species of soil, not a woodland one, and it is not associated with a stump or with buried wood.

Reading the Cap

Young caps are hemispherical and remarkably heavy, with the margin curved well under. They expand to broadly convex and finally flat, commonly 10-25 cm across and sometimes considerably more. The surface is dry, never viscid, and white to cream, ageing to a pale buff or faintly ochre tone especially where handled or sun-exposed.

The plating develops as the cap expands and the skin cannot keep up. Instead of the fine silky fibrils of a smooth-capped species, you get broad, flat, appressed scales with straight-edged margins, often arranged more or less concentrically, larger and more separated toward the rim and tighter over the disc. In humid conditions the plates may stay nearly closed and the cap looks almost smooth; in dry, breezy sites the cracking is dramatic. Rub the cap gently with a thumb and watch for a slow pale yellow-ochre stain, which is typical of this group and should be a gentle discolouration rather than an instant vivid flare.

Gills and the Spore Print

The gills are free, leaving a clear collar around the stipe apex, and they are crowded, narrow and thin relative to the enormous flesh above them. Colour progression is the key: pale greyish-pink in the unexpanded button, then dull pink, then purplish brown, then dark brown at maturity. In a big specimen the gills are frequently already dark by the time the cap has flattened, so check a younger fruit body from the same troop if you want to see the pink stage.

For the print, remove a cap of intermediate age, cut the stipe flush, and set it gill-down on card under a bowl for four to six hours. The deposit is a heavy dark brown. Because the whole mushroom is white and rather Amanita-shaped when young, this print is the single most decisive test you can run: a white deposit would take you into an entirely different part of the mushroom world, and a green one into Chlorophyllum.

Stipe, Ring and Base

The stipe is short and massive relative to the cap, roughly 6-18 cm long and 2-5 cm thick, white, smooth or slightly floccose below the ring, and typically equal or thickened downward into a blunt club rather than tapering. It is solid and firm right through, and it snaps cleanly rather than tearing into fibres.

The ring is large, high on the stipe and pendant, thick and membranous, and its underside often carries soft cottony patches or a coarse radiating pattern of veil remnants, a feature shared across the horse mushroom group. It is a single skirt, not a double sheathing collar, and it does not slide freely.

Dig the base out rather than pulling. There should be no sac, no free-edged cup and no ring of scaly bands around the base, only white mycelial threads binding grass roots and soil.

Flesh and Colour Change

Split a fruit body from cap edge to stipe base with a clean knife. The flesh is very thick, firm and white. Watch it for five minutes. In A. crocodilinus the change is slight and slow: a pale yellowish or ochre tinge, sometimes only where the knife has bruised the tissue, sometimes not at all. The stipe base should not flare bright chrome-yellow in the first few seconds. The odour is faint and pleasant, often with a slight aniseed or almond note in fresh material, and never harsh or inky.

Substrate and Season

This is a saprotroph of open, grassy ground: coastal terraces, dunes, pasture, prairie remnants, hedgerow gaps and the grassy edges of tracks. It fruits from late summer into autumn, driven by the first substantial rains or by prolonged coastal fog drip, and it is a species of nutrient-rich turf with a long-established mycelium, which is why arcs and partial rings are common. The absence of any tree association is itself an identification character, and finding a large white plated Agaricus in deep woodland litter should make you reconsider.

Separating the Look-alikes

Agaricus arvensis shares the grassland habitat, the pendant ring with cogwheel patterning and the pale yellowing, but it is smaller, its cap stays silky-smooth or only finely cracked rather than breaking into broad plates, and the yellow reaction on rubbing is usually stronger and quicker.

Agaricus augustus has genuine brown fibrillose scales on a yellow-ochre ground, a taller and more slender build, and it grows in woodland, on path edges and under trees rather than in open turf.

Agaricus campestris is much smaller and thinner-fleshed, with a simple fragile ring, bright pink young gills and no yellowing at all.

Agaricus bitorquis appears on compacted ground and roadsides with a smooth white cap and a distinctive double ring that sheathes the stipe with an upturned lower rim.

Agaricus xanthodermus is the important morphological contrast: its cut stipe base flushes vivid chrome-yellow within seconds, its odour is sharply phenolic, and its cap is smoother and often more angular in outline.

Amanita species can look similar as white buttons in grass, but they have a white spore print, gills that never pass through pink to dark brown, and a bulb that is enclosed in a volval sac or bordered by a rim, plus a ring that is thinner and often striate above.

Chlorophyllum and Leucoagaricus species also grow in turf with scaly white caps, but their gills stay white or turn greyish-green, and their spore prints are white, cream or dull green rather than dark brown.

A Working Sequence

Photograph the plated cap in place and note the open-ground habitat. Look for a younger fruit body to check pink gills. Cut one specimen lengthwise, time the colour change, sniff the cut flesh, and excavate the base to confirm there is no volva. Then take a dark brown print. Size, plating, open grassland, a thick pendant ring, slow yellowing and a dark brown deposit together make a confident record.