The diatom genus Humidophila (Bacillariophyta) in an historic Delogne sample with the description of 2 new species

During the analysis of a voucher sample from Frahan (Belgium) collected by the Belgian diatomist Charles-Henri Delogne, seven species from the genus Humidophila (Bacillariophyta) have been observed. Two species could not be identified using the currently available literature and are described as new to science. Humidophila delognei sp. nov. has always been identified in the past as H. contenta, but based on recent analysis of the original type material of the latter, it was clear that the currently accepted concept of the species was wrong. The new species is characterised by parallel valve margins, a linear valve outline, a distinct central fascia and T- or Y-shaped raphe endings. The other new species, H. aspera sp. nov., has undulating valve margins and distinct, though irregular siliceous markings in the central and axial area. The sample from Frahan was also indicated in 1885 by Henri Van Heurck as being the original material for the newly described species Humidophila (Navicula) flotowii, long considered a synonym of H. perpusilla. Both species can, however, be separated based on a different length/width ratio, with H. flotowii showing more slender, elongated valves. Other Humidophila species in the sample include H. contenta, H. laevissima, H. perpusilla and H. simplex. Navicula fragilarioides, a new name once introduced for Fragilaria laevissima, the basionym of H. laevissima, is lectotypified based on an original Krasske slide.