Species may be divided into three major sections: I. Species with a coloured cortex bordering the abhymenial surface. This structure is readily seen in sections, being more deeply coloured than the associated intermediate tissue, and composed either of cemented intertwined hyphae or compacted parallel hyphae. It is almost always associated with abhymenial hairs (H. tabacina). II. Species without a coloured cortex but possessing a well-developed intermediate tissue formed from hyphae either arranged parallel with the hymenial layer, or intertwined. Abhymenial hairs are present in pileate species (H. attenuata), absent from those which are resupinate (H. rhabarbarina). III. Species without a cortex or well developed intermediate tissue. Setae arise directly from a compact layer of intertwined hyphae seated upon the substratum. Species are resupinate, usually firmly attached to the substratum (H. corrugata) and without abhymenial hairs. Transverse sections through pileate species of Section I show several well defined tissues: (1) a layer of abhymenial hairs, sometimes in thickness exceeding that of the context, arising from (2) a deeply coloured usually cemented cortex, (3) a broad intermediate tissue of hyphae either arranged parallel with the hymenium, or intertwined and upright; (4) the setal layer, and (5) hymenium and subhymenium. (1) Abhymenial Hairs. In pileate species these appear upon the pileus surface and may form a tomentum, be imbricate when the surface appears silky, be grouped into strigose tufts, or appear as raised or depressed bands which are usually radiate and concolorous or of different shades of brown. They may persist, as in most pileate species, or become denuded, as in H. obesa, when the dark brown cortex becomes exposed. In umbonate-sessile and resupinate species which possess a cortex, abhymenial hairs are also present, forming a loose weft lying upon the substratum, so that plants are usually loosely attached. Abhymenial hairs are present in pileate species without a cortex, placed in Section II, but absent from resupinate species of that section, and from all species of Section III. They are formed from skeletal hyphae of species with dimitic hyphal systems, and from generative hyphae of those with monomitic systems. (2) Cortex. Present in species placed under Section I, the cortex may be seen readily in transverse sections since hyphae of which it is composed are more deeply coloured than tissues of the intermediate layer, compacted, cemented, and either intertwined (H. tabacina) or parallel (H. villosa). The cortex is composed mainly of skeletal hyphae in species with a dimitic hyphal system, though generative hyphae may be intertwined with them, and of generative hyphae in species which possess a monomitic system. The cortex is absent from species placed under Sections II and III. A second colour zone may be seen beneath the subhymenium of most species of Section I. It is irregular in position and formation, and for the most part composed of more densely arranged intertwined hyphae which are seldom cemented. (3) Intermediate Tissue. The bulk of the context in species placed under Sections I and II is formed from the intermediate tissue. In pileate species it is composed of hyphae radiately arranged parallel with the hymenium; in apileate species hyphae may be parallel (H. mougeotii), or loosely intertwined and mainly upright (H. cinnamomea). Sometimes the setal layer is so copiously developed that the intermediate tissue is reduced to a narrow zone (H. plurimaesetae); its presence then may be indicated by the presence of abhymenial hairs. In species placed under Section III, the intermediate tissue is wanting, though sometimes basal hyphae if well developed in small areas may simulate it. Absence of abhymenial hairs indicates that such specimens belong to Section III. (4) Setal Layer. Setae may be confined to a single row arising from the subhymenium (H. gladiola); or, more commonly, arranged in 2–5 rows which overlap