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Distribution:

P. denticulata—Indian Ocean, Equatorial Atlantic Ocean (?) (Simonsen, 1974, probably the only records).

P. glacialis—northern cold water region to temperate, southern cold water region.

P. pentaportula—off Uruguayan and Brazilian coasts, off Norwegian south coast (Syvertsen & Lange, 1990), Rhode Island (Hargraves, personal communication).

P. pseudodenticulata—southern cold water region. How to identify: The organic threads extruded from the processes scattered on the valve surface, connecting the cells in chains, are usually destroyed by fixation and preservation. Porosira spp. are therefore mainly present as single cells in preserved samples. The larger specimens may be misidentified as Coscinodicus spp. when observed in girdle view in water mounts. In valve view in water mounts, Porosira spp. are recognized by the numerous strutted processes, seen as dark spots. The large labiate process is also usually discernible, especially if the numerous small chloroplasts are destroyed. Processes and areolation are easily discernible on permanent mounts with LM.

Genus Skeletonema Greville 1865 (Plate 3, Table 6) Type: Skeletonema barbadense Greville.

References: Greville, 1866, p. 77, Plate 8, Figs. 3-6; Cleve, 1873a, p. 7; Cleve, 1900a, p. 22, Plate 8, Figs. 30 and 31; Cleve-Euler, 1912, p. 509,

Scale bar = 1 ju.m. Skeletonema costatum: (a) chain in girdle view, uppermost cell with two chloroplasts (note that the linking structures may be much shorter and less distinct); (b) intercalary valve with labiate process in the ring of marginal strutted processes; (c) separation valve (terminal/ end valve) with labiate processe close to valve center. Skeletonema menzellii: (a) chain in girdle view; (b) valve with long, thin external parts of strutted processes and labiate process close to the valve center. Scale bars = 10 ¿im except detail in P. pentaportula.

Species

Pervalvar axis (um)

Diameter (|im)

Marginal processes

S. costatum

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