Actinocyclus normanii

Valves are disc-shaped, flat to weakly concentrically undulate, with short mantles. Areolae are coarse and polygonal in shape, numbering 8-12 in 10 µm. Areolae often appear to radiate in pseudosectored, or fasciculate groups, of striae from the valve center. Several rimoportulae are located along the valve margin and are generally visible in LM. Rimoportulae are evenly spaced and number 4-8 per valve. A pseudonodule is located at the valve / mantle interface, but is often not visible due to its position.

In SEM images, the hexagonal to polygonal areolae and are occluded by flat external cribra. Externally, the opening of the rimoportula and of the pseudonodule are evident. Internally, the rimoportulae have long stalks, external openings are simple.

Some workers recognize the smaller form, A. normanii f. subsalsa (Juhl.-Dannf.) Hustedt from freshwater habitats (Hustedt 1957, Hasle 1977, Stoermer 1993). This form is recognized as differing from the nominate form in size and autecology (Hasle, 1977), with A. normanii f. subsalsa being 16-44 µm in diameter versus 30-110 µm for the nominate form, and the forma subsalsa occurring in fresh, inland waters (Hustedt 1927-1966, Hasle 1977, Stoermer et al. 1993). Although Hasle (1977) was not able to identify morphological discontinuities between the two forms, she and others (Belcher and Swale 1977) recognize that differentiating the two forms (or at least documenting the size distribution within collections) could provide ecological information.