Primary specific characters should include only those features that are constant, and not materially modified by geographical or exceptional local conditions as to environment, etc., and such alone should constitute a specific diagnosis. Trivial or local characteristics should follow the essential diagnosis as a rider. With the introduction of characters due to local conditions, a description ceases to be a specific diagnosis in the proper sense, and is merely a description of a certain state of a species occurring under a given set of conditions. Such, however, is too frequently the nature of so-called specific characters given in local Floras. A lengthy and laboured description suggests lack of power to grip essentials on the part of the compiler, and does not as a rule facilitate the recognition of the species intended. The idea of this work is to give the essential characters of each species as presented by pileus, gills, stem and spores respectively. This, however, owing to the views of some authors as to the predominant value of one feature to the exclusion of others, has not always been possible. For this reason the species of Britzelmayr are not included, as his descriptions, even when aided by what are presumably intended as figures, could not in many instances be construed into an intelligible diagnosis. In Cooke and Quelet's _Clavis Synoptica Hymenomycetum Europaeorum_ (1878), 1943 species of Agarics are recorded as European, and of these 939 are given as British. The present work includes descriptions of 2750 European species, of which 1553 are British, a number considerably in excess of that recorded for the species of any other European country. The species indicated by brackets have not been recorded as British. G. M. KEW, _June, 1902_. AGARICACEAE, Fries. Hymenium covering the entire surface of radiating lamellae or gills. Spores one-celled. Primary divisions of the Agaricaceae depend on the colour of the spores as seen in the mass, when deposited on an opaque substance; and not as seen under the microscope by transmitted light. I. LEUCOSPORAE. Spores white. In some cases with a tinge of yellow or pink. In some species of _Russula_ the spores are clear ochre, but such species belong obviously to _Russula_ notwithstanding. II. CHLOROSPORAE. Spores clear green or bluish-green. III. RHODOSPORAE. Spores salmon-colour or pink. IV. OCHROSPORAE. Spores ochraceous or rust-colour. V. MELANOSPORAE. Spores black, blackish-brown, or black with a purple tinge. LEUCOSPORAE. I. HAPLOPHYLLAE. Margin of gills acute; i.e. not longitudinally split or grooved. * MOLLES. More or less fleshy, soft, and soon decaying or shrivelling (not corky, woody, or rigid). + _Ring, or volva, or both present, or gills free._