Typification and diagnosis of Weissia × mittenii (Bruch & Schimp.) Mitt. emend. A.J.E.Sm. (Weissia multicapsularis × W. rostellata) (Pottiaceae, Bryophyta)

Introduction. Weissia × mittenii (Bruch & Schimp.) Mitt. emend. A.J.E.Sm. (Weissia multicapsularis × W. rostellata) has only been reported from five localities globally, all in south-east England, and was last found in 1920. The purpose of this paper is to typify the name W. × mittenii, provide a description of this hybrid taxon and clarify its diagnostic features.Methods. The main British herbaria were consulted, as well as the herbarium of William Mitten (NY). Variation in morphological traits across specimens was reviewed and compared with related species.Results. A lectotype and syntypes are designated, and a description and diagnosis provided.Conclusions. The key feature that characterises Weissia × mittenii is large variation in spore size in a single capsule, a range of 5–28 μm vs. 3–5 μm in similar species. Other characters of the gametophyte and sporophyte can also help identification. Further work is needed to confidently determine parentage.