Note. Numbers in parentheses occasionally found.

Note. Numbers in parentheses occasionally found.

The presence or absence of raphe fins.

The presence or absence of reinforced striae at the central area. Coarseness of valve striation.

Membraneis cballengeri Grunow, in Cleve 8c Grunow Paddock (Plate 64, Table 72)

Basionym: Navicula cballengeri Grunow in Cleve 8c Grunow. Synonym: Tropidoneis antarctica (Grunow in Cleve 8c Möller) Cleve (for other synonyms see Paddock, 1988, p. 81).

References: Cleve 8c Grunow, 1880, p. 64; Cleve, 1894b, p. 24; Paddock, 1988, p. 81, Plate 29; Paddock, 1990, p. 153, Fig. 11. Valve outline in girdle view a smooth curve from end to end. Raphe fins present about one-eighth of the valve length from the ends of the valve. Enlarged helictoglossae. Reinforced striae at central nodule varying in number and development. Flat unequal parts of valve face. Punctate valve striae just visible with LM.

Membraneis imposter Paddock (Table 72)

Reference: Paddock, 1988, p. 84, Plate 30; Paddock, 1990, p. 153, Figs. 12 and 13.

Raphe ridge in girdle view convex and slightly depressed at central nodule, poles sloping, slightly concave shoulder at about one-eighth of valve's length from each pole. No raphe fins and no reinforced striae at the central area. The two parts of valve face markedly unequal. Valve striae coarsely punctate.

2b. Valves lying in girdle or valve view; valves with only a low ridge.

Genus Manguinea Paddock 1988

Type: Manguinea fusiformis (Manguin) Paddock.

Generic characters:

Valves long and narrow with unequal faces.

Ridge not lobed.

Raphe fins or similar structures present.

Valve striae with separate puncta delicate and often difficult to see with LM.

Characters showing differences between species: Valve outline in girdle view.

Size of raphe fins.

Size of helictoglossae.

Manguinea fusiformis

Plagiotropis gaussii

PLATE 65 Manguinea fusiformis: cell in girdle view and enlarged helictoglossae and fin like structures. After Paddock (1988). Manguinea rigida: cell in girdle view with small raphe fins. After Paddock (1988) Plagiotropis gaussii: Cell in girdle view. After Paddock (1988). Scale bars = 10 p.m

Mattguinea fusifomtis (Manguin) Paddock (Plate 65, Table 72) Basionym: Tropidoneis fusiformis Manguin.

References: Manguin, 1957, p. 130, Plate 6, Fig. 39; Paddock, 1988, p. 88, Plate 32; Paddock, 1990, p. 153, Fig. 14. Frustule fusiform in girdle view. Valve outline slightly narrowed before poles, poles appearing slightly attenuated. Raphe fin-like structures positioned in the narrow parts. Enlarged helictoglossae.

Manguinea rigida (M. Peragallo) Paddock (Plate 65, Table 72) Basionym: Amphiprora rigida M. Peragallo. Synonym: Tropidoneis glacialis Heiden in Heiden & Kolbe. References: Van Heurck, 1909, Plate 1, Fig. 19; Peragallo, 1924, p. 21; Heiden & Kolbe, 1928, p. 656, Plate 5, Fig. 100; Paddock, 1988, p. 90, Plate 33; Paddock, 1990, p. 153, Fig. 15. Frustules in girdle view with parallel sides and bluntly rounded ends. Raphe fins small. Helictoglossae moderately sized.

Genus Plagiotropis Pfitzer 1871 emend. Paddock 1988 Type: Plagiotropis baltica Pfitzer.

The following species may be found in plankton:

Plagiotropis gaussii (Heiden, in Heiden & Kolbe) Paddock (Plate 65, Table 72)

Basionym: Tropidoneis gaussii Heiden in Heiden &c Kolbe. References: Heiden & Kolbe, 1928, p. 656, Plate 5, Fig. 102; Paddock, 1988, p. 63, Plate 24; Paddock, 1990, p. 152, Figs. 5 and 6. Valves delicate with narrow transapical axis and lying in girdle view. Whole frustules slightly waisted. Valve ridge very low and gently curving. Valve outline narrowed to a slight shoulder some distance before the valve poles. Raphe fins absent. Helictoglossae unusually large. Punctate transverse valve striae. Puncta visible with LM. Distribution: Southern cold water region.

How to identify: Most of these species are easier to identify in girdle than in valve view. Acid-cleaned material studied with phase or interference contrast is recommended.

Remarks: Some of these species are similar to Amphiprora spp. (= Entomoneis Ehrenberg in Patrick & Reimer, 1975) in gross morphology. Amphiprora occurs on brackish and marine sediments, occasionally in freshwater (Round et al., 1990); some species referred to Amphiprora in the past are extremely abundant in connection with ice in the Arctic and the Antarctic. Amphiprora and Tropidoneis have usually been regarded as closely related; EM observations disproved this, the main distinction being the internal structure of the keel of the valve (Paddock &c Sims, 1981).

D. Genus Pachyneis Simonsen 1974

Type: Pachyneis gerlachii Simonsen. Monospecific genus.

Pachyneis gerlachii Simonsen

References: Simonsen, 1974, p. 49, Plates 33 and 34; Paddock, 1986, p. 94, Figs. 19 and 20. Girdle view: Cells almost elliptical.

Valve view: Fusiform and highly vaulted. Valve membrane extremely delicate with a varying number of longitudinal folds, two to four on each side of the raphe. Raphe straight except for arching sideways near the central nodule. Vane-like structures (raphe fins?) near the poles. Striae parallel (interference contrast). Morphometric data: Apical axis, 100-370 /un; transapical axis, ca. 25-80 /u,m; 30-32 transapical striae in 10 jam. Distribution: Warm water region.

How to identify: The most characteristic feature is probably the longitudinal folds; high contrast and high resolution are needed to reveal the striation.

Family Bacillariaceae Ehrenberg 1831

The genera dealt with here were all placed in Nitzschiaceae Grunow 1860 by Simonsen (1979) and, with the exception of one genus, also by Glezer et al. (1988), whereas Round et al. (1990) used the older name Bacillariaceae. Only a few of the 15 genera included in Bacillariaceae sensu Round et al. are represented in marine plankton. Bacillaria paxillifera appears occasionally in plankton of shallow waters being swept up from the bottom. Cylindrotheca has its main distribution in and on mud; however, C. closterium (= Nitzschia closterium) has been recorded from a variety of habitats, including marine plankton. Neodenticula has several fossil but only one living marine planktonic species. Nitzschia had around 900 nomenclaturally valid species at the time when Mann (1986) wrote the paper "Nitzschia subgenus Nitzschia (Notes for a monograph of the Bacillariaceae, 2)." Of special interest to marine planktologists is the fact that his EM observations of the subgenus, including the generitype N. sigmoidea (Nitzsch) W. Smith, indicate that most of the marine planktonic so-called Nitzschia species are probably too remote morphologically from the generitype to fit into the genus. This is especially true for Fragilariopsis and Pseudo-nitzschia which in this chapter will be treated as separate genera.

Terminology specific to Bacillariaceae (Anonymous, 1975; Ross et al., 1979; Mann, 1978): (Fig. 18)

Canal raphe system consisting of

Raphe canal—a space on the inner side of the raphe cut off to a greater or lesser extent from the rest of the interior of the frustule. Fibula—a bridge of silica between portions of the valve on either side of the raphe (= keel punctum).

Interspace—the space between two fibulae.

Central interspace—the space between the two central fibulae.

Keel—the summit of the ridge bearing the raphe.

A central interspace larger than the others usually indicates the presence of two raphe slits, i.e., the presence of central raphe endings and a central nodule. Since a larger central interspace is observed with LM, but not always the corresponding central raphe endings and a central nodule, "central larger interspace" is a repeatedly used term in this chapter. In the past the wording was "die beiden mittleren Kielpunkte weiter voneinander entfernt" (e.g., Hustedt, 1958a) and "the two keel puncta in the middle more widely spaced than the others" (e.g., Hasle, 1965a,b). It should also be noted that the same authors used the term "pseudonodulus" instead of "central nodule" for the Bacillariaceae.

Characters of the genera dealt with:

Cells in chains of various types or more seldom solitary.

Cells rectangular or spindle shaped in girdle view.

Valves elongate although variable in outline.

Raphe with bridges of silica cross-linking the valve beneath the raphe ("canal raphe").

Raphe usually strongly eccentric running along one valve margin. Chloroplasts usually two plates, one toward each pole of the cell. Resting spores uncommon.

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