Guide to Meridion

Valves of Meridion are variable in symmetry to the transverse axis; they are either strongly heteropolar or isopolar. The heteropolar species are clavate (club-shaped) in valve view and wedge-shaped in girdle view. Septae (attached to girdle elements) are present. An apical porefield is present at one end. Valves may have 1-2 rimoportulae. Cells are joined together to form fan-shaped colonies. Chloroplasts are multiple and plate-like in living cells.

Meridion includes the common species, M. circulare and M. anceps. Meridion and Diatoma are closely related, and the features that distinguish them are based on ultrastructure of the copulae, or girdle bands (Williams 1985). Williams states that Meridion possesses 1) an intense poroidal area at each central ligula portion of the valvocopula, 2) an extended abvalvar process (generally), acting as a functional antiligula, and 3) absence of a ligula attachment at the valvocopula polar end.