Examination of four collections of Psathyrella frustulenta sensu A. H. Smith and four of P. frustulenta sensu Romagn. revealed that the former species is distinguished from the latter by its strongly developed veil and habitat (woods) and that there are considerable microscopic differences between the two (spore size, number and shape of pleurocystidia, pattern of cellular lining of gill edge). The former species is to be regarded as conspecific with Agaricus frustulentus Fries, the latter as conspecific with P. clivensis (Berk. & Br.) P. D. Orton. It is argued that the name P. fibrillosa was misapplied by J. E. Lange and A. H. Smith to a species for which the name P. friesii is introduced. Descriptions of P. friesii, P. frustulenta, P. clivensis, and P. obtusata are given.

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Journal Persoonia - Molecular Phylogeny and Evolution of Fungi
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Kits van Waveren, E. (1977). Notes on the genus Psathyrella—VI. Four controversial species of Psathyrella: P. fibrillosa, P. frustulenta, P. clivensis, and P. obtusata. Persoonia - Molecular Phylogeny and Evolution of Fungi, 9(3), 281–304.