Algae World: Sellaphora

Description

In the 'laevissima group' the valves generally have a simple linear outline, with broad rounded poles. There are no polar bars. The axial area is narrow, and the central area is ill-defined and ± rectangular or ± rounded. The axial area and adjacent parts of the striae may be depressed below the rest of the valve face but there is no conopeum, only a longitudinal groove. The effect in the LM is to create two grey longitudinal areas running parallel to the raphe slits, from near the centre to the poles, or no areas or lines.

The narrow axial area and ill-defined central areas separate the 'laevissima group' from the 'americana group'. The absence of polar bars separates the 'laevissima group from the 'pupula group'. The ill-defined central area, absence of conopea and absence of polar bars separate the 'laevissima group' from the 'bacillum group'.