Abstract
A new combination in the genus Planothidium is proposed. The identity of Planothidium lagerheimii (Cleve) C.E.Wetzel & Ector comb. nov. (≡ Cocconeis lagerheimii Cleve), from South America is revisited. This rather common species often reported from freshwater and aerial environments was identified in previous studies either as Cocconeis rudis Frenguelli or Planothidium salvadorianum (Hust.) Lange-Bert. (≡ Achnanthes salvadoriana Hust.). After having measured the original images and based on photographs gathered in the literature we concluded that the three species, Cocconeis lagerheimii, Cocconeis rudis and Achnanthes salvadoriana, belong to the same taxon. Following the International Code of Nomencalture the correct name is the earliest legitimate one with the same rank. Thus the species having priority should be Cleve’s species described in 1894. Freshwater material from southern Brazil was investigated to illustrate ultrastructural details.