Algae World: Sellaphora

Description

Members of the 'americana group' generally have a linear or linear-elliptical outline, with broad rounded poles; in one, the poles are broadly acute. They may be very slightly constricted at the centre. There are no polar bars. The axial area is wide (ax), and the central area is rounded, with well defined edges. The axial area is depressed below the rest of the valve face and covered by a broad conopeum ('canopy'), which is attached to the raphe-sternum on one side and with a free margin on the other side (Sims & Paddock 1979). The conopeum thus creates a flat longitudinal chamber or canal on the outside of the valve, next to the raphe system. The effect in the LM is to create two closely associated longitudinal lines running parallel to the raphe slits (arrows) along the whole length of the valve, becoming further apart where they enclose the central raphe endings.

The absence of polar bars and the wide axial area separate the 'americana group' from the 'bacillum group'. The shape of the central area, the wide axial area and the full-length conopea (creating canals running from pole to pole) separate the 'americana group' from the 'pupula group'. The wide axial area and full-length conopea separate the 'americana group' from the 'laevissima group.