_General Information_: This fungus is recognised by the dark fibrils on the cap, the 2-spored basidia easily seen with the low power of a microscope, and the pink gills when young. Much confusion has existed over this fungus and its nearest relatives. It is similar to the ‘Cultivated mushroom’, _A. hortensis_ (Cooke) Pilát, which is offered for sale in shops. However, it differs in several minor details and it may be that _A. bisporus_ is the fungus from which the cultivated mushroom developed, very probably unconsciously by man, but the history of the cultivated mushroom is very obscure. The cultivated mushroom when bought in British shops is white but in the United States two varieties are sold, one with the brownish fibrils predominating and a snow-white one where the fibrils do not darken; the former is frequently found in Europe. The white form is sometimes found in gardens where spent-mushroom spawn is used as mulching around fruit-trees but it has a rounder cap than _A. bisporus_. The cultivated mushroom accounts for an annual income of £14 million in the British Isles. _Illustrations_: _A. hortensis_--LH 133 (as the forma _albida_); NB 31⁷; WD 71¹. _A. bisporus_--Hvass 161; LH 133. [Illustration: Plate 43. Fleshy fungi: Spores purple-brown and borne on gills] B. BRACKET-FUNGI AND THEIR RELATIVES _Key to major genera_ A group of fungi which includes the bracket fungi, hedgehog fungi, fairy-clubs and their relatives; in the majority of species the margin continues to grow through the favourable part of the season and so often envelopes leaves, grass, etc. 1. Spore-bearing layer (hymenium) quite smooth, spread over veins or shallow pores; fruit-body top-shaped, fan-shaped or club-shaped, or spread over the substrate (resupinate) 2 Spore-bearing layer lining the inner surface of tubes or borne on warts or spines 17 2. Fruit-body club-shaped, coral-shaped or distinctly funnel-shaped, fan-like or resembling an agaric 3 Fruit-body resupinate or with poorly developed cap 11 3. Fruit-body coral-like or club-shaped with clubs grouped or branched 4 Fruit-body resembling an agaric or funnel-shaped to fan-shaped 9 4. Fruit-body large, branched with flattened and curled lobes and so resembling a cauliflower _Sparassis_ Fruit-body of single or grouped clubs or if branched then not resembling a cauliflower, the lobes being cylindrical or only slightly flattened and hardly bent 5 5. Fruit-body small arising from a seed-like structure or growing attached to dead herbaceous plant remains 6