A new mainland population of the rare European endemic Acaulon piligerum (Pottiaceae)

The genus Acaulon Müll.Hal. (Pottiaceae) is represented in Europe by seven species of minute, bulbi-form, ephemeral terricolous mosses (Hodgetts and Lockhart 2020). Of these, the Mediterranean endemic A. piligerum (De Not.) Limpr. is considered one of the rarest and most elusive European species (Cogoni et al. 1999), and the only one with lamellae on the adaxial side of the nerve (Sérgio 1992). This trait is characteristic of subgenus Alaticosta Stone (Stone 1976), which includes only two other southern-hemi-sphere species, A. chrysacanthum I.G.Stone, from Australia, and A. leucochaete I.G.Stone, from Australia and South Africa (Magill 1981). Despite the fact that A. piligerum was described as a variety of Phascum bul-bosum Voit. more than a century earlier than the Australian species, the original description did not report the presence of adaxial lamellae (De Notaris 1838), and neither was this remarkable character noticed when it was reassigned to Acaulon by Limpricht (1885). This led to further misinterpretation of the species as a Mediterranean form of Acaulon triquetrum (Spruce) Müll.Hal. (Limpricht 1885), a widespread species which lacks adaxial lamellae and was later included as a synonym of A. triquetrum (Corley et al. 1981). A decade later, Sérgio (1992), during a revision of the type material of A. piligerum , was the fi rst to note the