On the type of Coprinus rhombisporus
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Original description
Coprinus rhombisporus P.D. Orton in Notes R. bot. Gdn Edinb. 32: 145. 1972 (Notes on British agarics IV). Holotype: England: Norfolk, Surlingham, Wheatfen Broad, 3 Aug. 1968, P.D. Orton 3249 (E).

A sociis a sporis et habitatione distinguitur.
Pileus primo ovoideus vel conicus 3-10/4-7 mm, dein expansus fere planus 7-12 mm, interdum leviter umbonatus vel ad marginem revolutus, primo albus et aliquantum crasse albo, griseo, ochraceo vel luteolofusco squamulosus, dein griseascens striatus fere laevis nisi ad discum pallidiore persistentiore squamulosum. Lamellae liberae vel anguste adnatae, ex albo pallide umbrinae vel violaceofuscae, postremo nigricantes, ad aciem primo alboflocculosae. Stipes 25-50/0.5-1.5 mm, aequalis vel sursum attenuatus, ad basim saepe leviter incrassatus, albus vel hyalino-albus, ad apicem fere laevis deorsum albopruinosus vel squamulosus, ad basim albotomentosus vel strigosus. Caro tenuissima, hyalina, ad discum albida. Odor nullus. Sporae lentiformes, ellipsoideae aut fere rhomboideae vel obscure angulato-amygdaliformes. 7-9/5-5.5/5.5-6.5 µm, in cumulo umbrinae vel obscure cacainae. Basidia 4-sporigera. Cystidia aciei lamellarum sphaeropedunculata, vesiculosa vel clavata, 10-22 µm lata, rariore late lageniformia 30-60/16-26 µm. Cystidia faciei lamellarum cylindrico-vesiculosa, 60-90/14-24 µm. Cellulae veli filamentosi angustae, 3-8 µm latae, diverticulis 1-2 µm latis ornatae, tunicis 0.5-1 µm latis.
Norfolk. Surlingham, Wheatfen Broad, ad reliqua et caules graminis et caricis, 18 vii 1956, 27 vii 1960, 3 viii 1968, Orton 3249 (holo. E). Devon. Membury, 10 viii 1971.

Cap ± ovoid or conical at first, 3-10/4-7 mm, then expanded ± plane or slightly umbonate, 7-11 mm broad, sometimes becoming revolute at margin, white with rather thick white to greyish, ochraceous or buff fibrillose scales at first especially at centre, then greyish striate and ± smooth except at often more persistently scaly and paler centre. Gills free or narrowly adnate, white then pale umber or violaceous-date, finally blackish, edge white flocculose at first. Stem 25-50/0.5-1.5 mm, equal or slightly attenuated upwards often with slightly swollen base, white or hyaline-white, ± smooth above but white pruinose or with scales as on cap towards white tomentose or strigose base. Flesh very thin, hyaline-whitish in centre of cap. Smell none. Spore-print umber or dark chocolate. Spores lentiform, ellipsoid in side-view, ± rhomboid or indistinctly angled-amygdaliform in face-view, 7-9/5-5.5/5.5-6.5 µm. Basidia 4-spored. Marginal cystidia sphaeropedunculate, vesiculose or clavate, 10-22 µm in diam., more rarely broadly lageniform 30-60/16-26 µm. Facial cystidia cylindric-vesiculose, 60-90/14-24 µm. Hyphae of veil on cap narrow, 3-8 µm in diam., walls 0.5-1 µm thick with diverticulae 1-2 µm in diam. Norfolk. Surlingham, Wheatfen Broad, on grass or Carex debris, 18 vii 1956, 17 vii 1960, 3 viii 1968, Orton 3249 (holo. E); Devon. Membury, 10 viii 1971. Distinguished from its allies by spore shape and size and habitat. Distribution unknown, but it is fairly common in the Norfolk locality especially in the summer. This species and C. xantholepis described below belong to a group within the section Impexi characterised by growing in herbaceous debris and producing rather small fruit-bodies with a veil of relatively narrow often diverticulate and sometimes thick-walled hyphae. The members of this group appear to show some diversity of spore shape, although as yet only C. friesii and C. tigrinellus have spores sometimes exceeding 9 µm in length, and what appear to be perisporial remnants are sometimes seen on the spores of one or two species. Further observations on exact habitat, colour of the veil scales on the cap, and whether the hyphae of the veil scales are thick-walled or not, are needed to clarify this group. For the moment I prefer to regard those with differently shaped spores as separate species. Pilat & Svrcek have published a key to this group (Ceske Mykol. 21, 3: 136-145, 1967), but the characters they give for their new species are different from those of my British finds, hence the description of these new species. Although the hyphae of the veil scales are rather broader, C. argenteus (described above) also belongs to this group. So far, therefore, we have in Britain argenteus, friesii, saichiae and urticaecola with white scales on the cap, differing mainly in spore details, tigrinellis with blackish or dark brown scales and relatively large spores, and rhombisporus and xantholepis with ochraceous or buff scales also differing apparently mainly in spore size. An as yet undescribed species found on three occasions in Hertfordshire with small peculiarly shaped spores apparently with perisporial remnants in places remains to be described when further material has been found.

Observations:
Spores [20,1,1] 6.1--9.2 x 5.0--8.2 µm, red-brown, ovoid or ellipsoid; Q= 1.10--1.35, av. Q= 1.20; av. L= 7.8, av. B= 6.5 µm. Germ pore central. Veil 3--7 µm wide, thick-walled, walls up to 2.5 µm thick, pale yellow-brown. Clamp-connections not observed.

Notes: the breadth of the spores was found to be somewhat greater than cited by Orton and the thickness that he gave for the walls of the veil (0.5--1 µm) did not agree with our observations. In the poorly preserved type material, these were the only features that could be examined.

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