Achnanthidium subhudsonis var. kraeuselii

Valves are narrow and lanceolate. The raphe valve has a lanceolate axial area becoming narrow at the apices. The raphe is expanded near the valve center and the external proximal raphe ends have expanded pores. Internally, the proximal raphe ends are slighly deflected to the opposite side from the terminal fissures. The terminal raphe fissures are deflected to the same side and extend over the valve mantle. Striae are radiate and more broadly spaced at the central valve than near the ends. The rapheless valve is flexed about the apical axis. The rapheless valve also has a more narrow lanceolate axial area than the raphe valve. The axial area of the rapheless valve is asymmetrically bent near the apices in many specimens. Striae are slightly radiate on the rapheless valve. The striae are composed of poroid areolae. The external and internal openings of the areolae are circular, or nearly so.

Note that Hustedt (1921) described the species, A. subhudsonis from tropical regions. This taxon is reported as having a higher stria density and parallel striae (as compared to radiate striae)(Simonsen 1987) than the specimens illustrated here. Cholnoky (1954) described A. kraeuselli and later included it as a variety, A. subhudsonis var. kraeuselii, the name we adopt here. We expect that future work with resolve the differences, if there are differences, between the nominate and other varieties. We also expect that this taxon will soon be included in a new genus; it is clearly not aligned with either Achnanthes as it was originally described (Cholnoky 1954), or Achnanthidium as it was later transferred (Kobayashi et al. 2006).