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The genus Cortinarius is the largest genus of agarics in the British Isles, indeed in Europe and North America—perhaps in the world. It includes some of our most beautiful agarics, yet it is one of the least satisfying to the mycologist because of the difficulties experienced in identifying collections—partly because many species are so seldom seen.

Cortinarius contains just under two hundred and fifty recognisable British species, although recent research has shown that many more are yet to be described from this country as new to science. Except for some very characteristic species the individual members within the genus Cortinarius are often very difficult to separate one from the other; however, Cortinarius is one of our least difficult genera to recognise in the field owing to the presence when mature of rust-coloured gills and a cobwebby veil which extends from the margin of the cap to the stem. This structure is termed a cortina (Fig. 14) and in young specimens covers the gills with delicate filaments. As the cap expands the cottony or cobwebby filaments are stretched and either disappear entirely or may collapse to form a ring-like zone of filaments on the stem. In some species a second completely enveloping veil is also found, and this veil is viscid in one distinct group of which C. pseudosalor already described is a member. The gills in the genus are variable in colour when young although constant for a single species; they may be lilaceous purple, orange, brown, red, yellow-ochraceous or tan, but ultimately in all members at maturity they become rust-colour. The spores under the microscope are richly coloured, yellow to red-brown and are frequently strongly warted; in mass they are rust-brown and this character coupled with the presence of the cobweb-like veil characterises the genus.

Within the genus Cortinarius there is a wide range of characters varying from species with distinctly sticky caps and stems, some with sticky caps and dry stems to those with both dry caps and stems. A few species are very large and fleshy whilst others are quite slender and many of the latter rapidly change colour on drying out and are then said to be hygrophanous. However, although there is such a large spectrum of characters in a single genus the species all have in common the cortina and rust-coloured gills, the latter often appearing as if powdered with rusty dust.

Utilising the characters mentioned above this very large genus can be split into the following six sections, called by the mycologist subgenera:

1 Large to medium sized fleshy agarics with viscid caps and stems—Myxacium

2 Large, fleshy agarics with viscid or tacky caps when fresh but dry stems—Phlegmacium

3 Large to medium sized agarics with dry, scaly or humid caps and dry stems which if orange tawny are robust—Cortinarius

4 Medium, rarely large, agarics with dry, silky to innately fibrillose caps, slender stems and frequently with at least part of the fruit-body yellow, orange or reddish—Dermocybe

5 Medium to small agarics with silky fibrillose, non-hygrophanous caps which may become tacky in wet weather and then usually with robust, clavate-bulbous stems—Sericeocybe

6 Small, less frequently medium or large agarics, all with distinctly hygrophanous caps—Hydrocybe.

In several continental books some or all of these divisions are recognised as distinct genera in their own right. The subgenus Telamonia which occurs in many texts was formerly thought to differ from Hydrocybe in the presence of a universal veil; the universal veil is a second veil which completely envelopes the fruit-body when it is young and is in addition to the cortina. However, the modern treatment would seem to suggest that the presence of the universal veil is not of the utmost importance and so the two subgenera are incorporated into one. The name Hydrocybe reflects the character of changing colour as it dries out because of the loss of water. Within each subgenus the species are distinguished by the colour of the young gills and of the cap, the veil colour and texture, and microscopic characters of the spores, particularly their size.

The majority of species of Cortinarius are mycorrhizal and like the boletes possess very specific relationships with tree species. Thus some are typical of coniferous woodland and others typical of deciduous woodland in general, whilst others typify woods of a particular tree, e.g. beech, oak, birch, pine, larch. Some species are characteristic of woods on limestone or chalky soils (calcareous) whilst others are characteristic of woods on sandy, heathy acidic soils. For example, Cortinarius armillatus (Fries) Fries which is found in damp woods and possesses one or more cinnabar-red or scarlet zones on the stem and red fibrils at the stem-base appears to be connected with birch. Several species are associated with native trees whilst others have undoubtedly been introduced from abroad. They are very important in the economy of the woodland ecosystem.

One of the most beautiful and easily distinguished of our British species is Cortinarius violaceus (Fries) Fries which has uniformly deep violet-coloured stem and cap and coloured cystidia on the gill-margin, a character unusual in Cortinarius.

No species are known to be truly poisonous and many species are known to be edible, but many are too small to be of any value. Some of the larger species are regarded as good to eat, but frequently are too scarce. Thus the necessity for experience to recognise the different species, coupled with their often unpleasant tastes make them an unimportant group of agarics for eating.