Key To Species

1 a. Cells usually solitary, occasionally connected by a thread from a central strutted process 2

lb. Cells in a matrix extruded from the girdle forming sheet like colonies . .

P. muriformis2 (Loeblich, Wight, 8c Darley) Round

2a. Girdle with a continuous wing P. sol (Wallich) Schutt

2b. Girdle with lobes P. blanda2 (A. Schmidt) Syvertsen 8c Hasle

Distribution: Warm water region (P. sol has also been recorded in Atlantic waters in the Norwegian Sea and along the Norwegian west coast). How to identify: Since the organic material attached to the girdle disappears during acid cleaning, Planktoniella spp. are easily misidentified as Thalassiosira spp. when cleaned, mounted valves are examined. Examination of water mounts is therefore more reliable. Remarks: Coscinodiscus blandus Schmidt (1878) as well as C. latimarginatus Guo (1981) were described with lobes attached to the girdle. The combination Thalassiosira blandus (sic!) Desikachary 8c Gowthaman

2 Basionyms: Coenobiodiscus muriformis Loeblich, Wight, & Darley and Coscinodiscus blandus A. Schmidt, respectively.

appeared in Desikachary (1989, p. 9), evidently based on LM observations. The valve gross morphology of C. blandus as resolved with SEM is much the same as those in Thalassiosira and Planktoniella. The justification for the combination Planktoniella blanda is the fact that organic extrusions from the girdle (in this case the lobes) are present in Planktoniella but not in Thalassiosira. Hallegraeff (1992) apparently regarded C. blandus A. Schmidt and C. bipartitus Rattray as two separate taxa and made the new combination, Thalassiosira bipartita (Rattray) Hallegraeff. Thalassiosira simonsenii Hasle 8c G. Fryxell has occluded processes but may even so belong to this genus, possibly as conspecific with P. blanda.

Genus Porosira Jorgensen 1905 (Plate 3, Table 5) Type: Porosira glacialis (Grunow) Jorgensen. Basionym: Podosira hormoides var. glacialis Grunow. References: Grunow, 1884, p. 108, Plate 5, Fig. 32; j0rgensen, 1905, p. 97, Plate 6, Fig. 7; Hustedt, 1930, p. 314, Fig. 153; Hustedt, 1958a, p. 117, Figs. 20 and 21; Jouse et al., 1962, p. 66, Plate 4, Figs. 15 and 17; Simonsen, 1974, p. 11, Plates 7 and 8; Hasle, 1973a, pp. 6-15, Figs. 4, 5, and 13-43; Takano, 1990, pp. 176-177; Syvertsen 8c Lange, 1990.

Generic characters:

Cells single or in loose chains.

Cells discoid.

A central annulus.

Radial areolation.

One large labiate process in the marginal zone. Numerous (strutted) processes all over the valve face.

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