In 1855, Smith described Achnanthidium lineare based on material collected in 1854 from Lasswade in Scotland. In his collection, a second entry for Achnanthidium lin- eare from 1854 is present, but based on material from the Vaucluse in southern France. Scanning electron microscopy analysis of both samples revealed that the Achnanthidium valves found in the Vaucluse sample represent A. pyrenaicum (huSt.) H. KobayaSi but that the Lasswade population clearly represents A. lineare s.s. For a long time, A. lineare was considered to be a synonym of A. minutissimum (Kütz.) Czarn. Based on the application of a narrower species concept, however, the analysis of the type material of A. lineare and A. minutissimum justifies the separation of both taxa. Additionally, several Achnanthidium minutissimum-populations from various European countries (France, Italy, Portugal, UK, Sweden) showing a clear morphological similarity to Smiths A. lineare from Lasswade, have been examined and their morphology is discussed compared to the Lasswade type material of A. lineare s.s. The analysis also indicated the presence of two new species: A. pseudolineare Van de VijVer, noVaiS et eCtor from Portugal and A. sublineare Van de VijVer, jarlman et eCtor from Sweden. Both species are described as new to science. Using ecological data from the different European populations, the ecological preferences of A. lineare in Europe have been defined. The morphological results of the type material analysis of A. lineare and of A. pseudolineare and A. sublineare were also compared with the available literature data on the type material of A. affine (Grunow) Czarn., A. jackii rabenh., A. lailae Van de VijVer in zidaroVa et al., A. minutissimum, and A. strictum e. reiChardt.
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