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b The higher numbers are found near the valve ends.

b The higher numbers are found near the valve ends.

Common characters:

Cells rectangular in girdle view.

Valves lanceolate and sometimes with slightly produced ends.

Transapical striae lineate (crossed by finer longitudinal striation).

Two chloroplasts per cell, one at each side of the girdle.

Navicula directa (W. Smith) Ralfs in Pritchard (Plate 63, Table 70)

Basionym: Pinnularia directa W. Smith.

References: Smith, 1853, p. 56, Plate 18, Fig. 172; Pritchard, 1861, p. 906;

Gran 8c Angst, 1931, p. 499, Fig. 87; Heimdal, 1970, Figs. 25-28. Valves narrow and lanceolate with subacute ends. Raphe-sternum indistinct. Striae parallel and uniformly spaced throughout the whole valve. Each chloroplast covering the girdle from end to end.

Navicula distans (W. Smith) Ralfs in Pritchard (Plate 63, Table 70)

Basionym: Pinnularia distans W. Smith.

References: Smith, 1853, p. 56, Plate 18, Fig. 169; Pritchard, 1861, p. 907;

Gran & Angst, 1931, p. 499, Fig. 86; Cupp, 1943, p. 193, Fig. 143. Valves lanceolate. Striae radiate. Each chloroplast covering less than half of each side of the girdle.

Navicula transitans var. derasa (Grunow, in Cleve 8c Grunow) Cleve

(Plate 63, Table 70)

Basionym: Navicula derasa Grunow in Cleve 8c Grunow.

References: Cleve 8c Grunow, 1880, p. 39, Plate 2, Fig. 46; Cleve, 1883, p.

467, Plate 36, Figs. 31, 33, and 37; Heimdal, 1970, Figs. 12-18. Valves lanceolate with slightly rostrate ends. Sternum narrow and not centrally expanded. Striae parallel to radiate, closer together near the ends than in the middle of the valves. Chloroplasts often asymmetrical and not covering the whole length of each side of the girdle.

Navícula transitans var. derasa f. delicatula Heimdal (Plate 63, Table 70) Reference: Heimdal, 1970, p. 72, Figs. 30-37. Valves lanceolate to elliptical, mostly with slightly rostrate ends. Striae parallel and uniformly spaced throughout the valve. Each of the two chloroplasts covering most of each side of the girdle. Distribution: Uncertain.

How to identify: Size and position of the chloroplasts are useful distinctive characters. Positive identification requires examination of cleaned valves mounted in a medium of a high refractive index. Due to the coarse silicification N. directa and N. distans are best examined in brightfield illumination.

B. Genus Pleurosigma W. Smith 1852

Type: Pleurosigma angulatum sensu W. Smith emend. Sterrenburg. Basionym: Navícula angulata Quekett pro parte quoad typum. Synonyms: See Sterrenburg, 1991a.

The genus was last monographed 100 years ago (Peragallo, 1891), and many more taxa have been introduced since then without a revision of the genus. Recently Cardinal et al. (1989) introduced criteria for species characterization, revealed by SEM, and Sterrenberg (1991b) pointed out LM criteria suitable for taxonomic purposes.

There are some 250 major taxonomic entries for Pleurosigma in VanLan-dingham (1978), 90 of them are listed as valid. According to the information given by Cupp (1943), Hendey (1964), and Simonsen (1974), less than one-tenth appear more or less regularly in the plankton. Two species are treated here, viz. Pleurosigma normattii, the most common and most widely spread of all Pleurosigma species, found from the tropics to the polar seas, often in the plankton (Hendey, 1964), and P. directum, which has been characterized as an "almost cosmopolitan plankton species" (Simonsen, 1974, p. 45). A third species, P. simonsenii, was described as a planktonic species from the Indian Ocean (Simonsen, 1974) and later recorded as abundant in the phyto-plankton of the western English Channel (Boalch &c Harbour, 1977).

Distinctive characters:

Valves more or less flattened, gently sigmoid, or almost straight.

Valve outline lanceolate.

Raphe straight or more or less sigmoid and central.

Three striae systems: one transverse and two oblique.

Two or four elongated chloroplasts, often extremely convoluted and lying under valve face rather than along girdle, many pyrenoids per chloroplast (Cox, 1981).

Characters showing differences between subgroups or species:

Shape of valve.

Curvature of raphe.

Crossing angle of striae.

Areolation at valve apices.

Color in standardized darkfield (Sterrenburg, 1991b).

Pleurosigma directum Grunow in Cleve 8c Grunow (Plate 62, Table 71) References: Cleve 8c Grunow, 1880, p. 53; Peragallo, 1891, p. 14, Plate 5, Fig. 29; Simonsen, 1974, p. 45, Plate 29, Fig. 2. Valves rhombo-lanceolate to elliptic-lanceolate. Raphe almost straight.

Pleurosigma normanii Ralfs in Pritchard (Plate 62, Table 71) References: Pritchard, 1861, p. 919; Cupp, 1943, p. 196, Fig. 148; Sterrenburg, 1991b, Fig. 2. Valves broadly lanceolate, slightly sigmoid, with subacute ends. Raphe nearly central, sigmoid with single curvature. Raphe-sternum or central nodule dilated transversely. Crossing angle of striae greater at center than toward valve apices. Color in standardized darkfield deep blue with silverish center (Sterrenburg, 1991b).

Pleurosigma simonsenii Hasle nom. nov. (Table 71) Synonym: Pleurosigma planctonicum Simonsen. References: Simonsen 1974, p. 46, Plate 30; Boalch 8c Harbour, 1977. Valves slightly sigmoid near the ends and flat; ends acute and not protracted. Raphe straight, sigmoid before the ends, central in the middle, eccentric before the ends, central pores close together. Raphe-sternum narrow and not centrally expanded. Valve thin and striation hardly visible with LM. No color in standardized darkfield (Sterrenburg, personal communication).

TABLE 71 Morphometric Data of Pleurosigma spp.

Striae in 10 |ira

Species

Transapical axis Apical axis (pm) (p.m)

Transverse Oblique

P. directum P. normanii P. simonsenii

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