Hymenophore a tenuous greyish film barely visible upon the substratum, annual, arachnoid, adnate, to 5 x 2 cm, surface clingy white, delicately pruinose, arachnoid; margin arachnoid, white, adnate. Context a tenuous layer 10-18 µ thick, basal layer of a few repent hyphae, soon collapsed, intermediate layer absent; generative hyphae 2-2.5 µ diameter, walls 0.2 µ thick, hyaline, naked, branched, septate, with clamp connexions. Hymenial layer composed of scattered basidia, paraphyses and cystidia. Basidia arising directly from the basal layer, clavate, 6-8 x 4-5 µ, 4-spored; sterigmata arcuate, 2-4 µ long. Paraphyses scanty, subclavate, about half the size of the basidia. Cystidia arising in the base of the context and projecting for the greater part of their length, cylindrical or slightly attenuated towards their apices, 45-64 x 4-6 µ, bases forked, apices rounded and inflated, lumen capillary, save at apex where expanded and bulbous, walls naked or as often enmeshed in delicate hyphal sheaths. Spores globose, apiculate, 3.5-4 µ diameter, walls smooth, hyaline, 0.2 µ thick.

DISTRIBUTION. New Zealand.

HABITAT. Effused on decayed decorticated logs.

Hymenophorum annuum, tenue, arachnoideum, adnatum; superficie griseo-alba, pruinosa, non rimosa. Hyphae contextus fibulatae, 2-2.5 µ diam., nudae. Basidia clavata, 6-8 x 4-5 µ, 4 sporis. Cystidia cylindricalia, eminentia, 45-64 x 4-6 µ, apice inflato, nudo vel in reticulo hypharum. Sporae globosae, 3-5-4 µ diam., laeves, hyalinae.

This delicate species may be identified by the cystidia, small basidia, small globose spores and absence of an intermediate layer. Cystidia project for almost their entire length and are cylindrical with inflated apices; walls, save at the apices, are so thickened that the lumen is capillary, and are destroyed with potassium hydroxide solutions. They become enmeshed in hyphal sheaths. Apices are thin-walled and inflated, so that the cystidia, like those of P. gracillima, resemble small inverted thermometers. Basidia are small, clavate, and develop directly from hyphae of the basal layer. Fructifications appear as a delicate white or greyish arachnoid film upon the surface of decayed logs; visible to the eye when fresh, in the herbarium fertile portions can be seen only with the aid of a dissecting microscope.
The species resembles P. accedens Bourd. & Galz. in the shape of the cystidia, but differs in. the smaller basidia, globose spores and more delicate fructifications.

Metrosideros robusta A. Cunn. Auckland: Hick's Bay, 300ft, May, 1952, G.H.C., type collection, P.D.D. herbarium, No. 11483.