![Sample our Environment & Agriculture journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5934/TOC-Subject-Sample-2021-Environment-Agriculture.jpg"})
Abstract
The intersection angle of the oblique striae in Pleurosigma, the difference in the densities of the longitudinal and transverse striae in Gyrosigma and the single or double curvature of the raphe sternum in both Gyrosigma and Pleurosigma permit distinction of subgroups in these genera. Four new light microscopical criteria: the colour in darkfield, the raphe angle, the apical details and the position of the terminal areas, are suitable for taxonomy. Additional light microscopical features are discussed. The findings are based on populations of different origins for 31 species. A new species, Pleurosigma curiosa van den Heuvel is described.