The genus Megacollybia, originally proposed for M. (Collybia) platyphylla, has traditionally been treated as monotaxic. A phylogenetic reconstruction based on ITS rDNA sequences indicates that several species are involved, with strong phylogeographic signal. Although morphological characters are largely qualitative, examination of basidiomata suggests that specimens included in discrete clades can be distinguished at the species level. On these bases (phylogenetic, morphological), several new taxa are proposed: M. clitocyboidea, M. texensis, M. fusca, M. subfurfuracea, M. rodmani (with f. murina) and M. marginata. Tricholomopsis fallax is transferred to Megacollybia; M. platyphylla remains the type species of the genus but appears to be restricted to Europe, Scandinavia and western and central Russia.
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