Cosmarium excavatum var. duplo-majus. Notice globose, granulate semicells mutually separated by an elongate isthmus.
Cell dimensions (L x B): 42 x 25 µm
C. excavatum var. duplo-majus in vertical view. Notice circular outline of semicell and isthmus.
C. excavatum, a medium-sized granulate Cosmarium, is marked by an elongate isthmus and almost globose semicells. Cell wall granules are relatively large and evenly distributed over the wall (but lacking at the isthmus). From the Netherlands only C. excavatum var. duplo-majus is known, being almost twice as large as the nominate variety and found in one single moorland pool, in 1926.
Image © Alfred van Geest (after Canadian material)
Another cell of C. excavatum var. duplo-majus in frontal view (focussed on cell outline).
Image © Jan Šťastný (after Irish material)
SEM picture of C. excavatum var. duplo-majus. Notice large, evenly distributed granules (each surrounded by a hexagon of pores)