Type studies of Russula species described by WA Murrill, 1. R. roseiisabellina, R. sericella, and R. obscuriformis

The study of the North American russulas remains severely handicapped by the lack of sufficient microscopic detail on approximately one third of the described taxa. In this contribution, microscopic features are examined and illustrated in detail for three of Murrill's type specimens. The authors reinterpret these data in combination with the macroscopic features supplied in the original diagnoses and propose a more appropriate infrageneric placement of the taxa: (i) R. roseiisabellina is not close to R. sericeonitens (subgenus Russula) as suggested in Murrill's protologue, nor is it a synonym of R. rimosa (subgenus Heterophyllidia) as suggested by Hesler - it is a typical member of the R. foetens group (subgenus Ingratula); (ii) R. sericella is absolutely unrelated to R. obscuriformis as suggested in Murrill's protologue and probably fits within subgenus Russula or Tenellula; and (iii) R. obscuriformis is similar to the recently described R. texensis in Russula sect. Xerampelinae, but new collections are needed to verify whether it possesses the typical field characters of this group.