Guide to Aneumastus

Species of the genus Aneumastus are typically lanceolate, with capitate or rostrate ends. The valve face is flat, with a narrow valve mantle. The striae are uniseriate over most of valve face, biseriate near the valve mantle and composed of complex areolae. The areolae open into deep pits internally, a feature visible in SEM. The raphe sternum is narrow, but widens centrally to form an elliptical or rectangular central area. The first girdle band, or valvocopulum, is modified and thickened. Living cells contain two plastids, which are H-shaped in girdle view.

Aneumastus is considered to be closely related to Mastogloia, but lacks the partecta of Mastogloia. Aneumastus differs from Navicula in possessing plastids composed of two plates, which appear H-shaped in girdle view. Until recently, species included in Aneumastus had been included in Navicula (Navicula tuscula group). This genus contains only a few species, broadly distributed across North America, but locally uncommon in occurrence. It grows in epipelic and epipsammic habitats of alkaline waters.