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Distribution: All three species are probably cosmopolitan. How to identify: If possible to see at all with LM, the striation can only be seen on cleaned valves mounted in a medium of a high refractive index or also occasionally as a dry mount. The large size and the extremely delicate valve striation characterize P. simonsenii.

Remarks: Pleurosigma planctonicum Simonsen is a later homonym of P. planctonicum Cleve-Euler (see Taxonomic Appendix). Gyrosigma Hassall, separated from Pleurosigma by having longditudinal and transverse (no oblique) striae, is another large genus with a few species occasionally occurring in marine and brackish water plankton, e.g., G. macrum (W. Smith) Griffith & Henfrey (Sterrenburg, personal communication).

C. The "Tropidoneis" group

Termimology mainly used for the Tropidoneis group (Paddock & Sims, 1981; Paddock, 1988).

Raphe ridge—a simple angular elevation of the valve which bears the raphe and raises it, but which lacks specialized supporting structures.

Valve face—the whole valve surface.

Greater and lesser parts of valve face—the two parts of valve face divided by the raphe, termed "greater face" and "lesser face," respectively, by Paddock (1988).

Raphe fins—paired external small vane-like (blade, plate-like) siliceous structures, each shaped as a shark's dorsal fin (SEM) and arising from the sternum (also present in Stauropsis).

Notes: Paddock & Sims (1981, p. 178): "In practice it proved extremely difficult to decide whether some diatoms have a raphe 'raised upon a keel' or whether the valve of the diatom which is 'highly vaulted' is merely laterally compressed to an extreme degree." Paddock (1988, p. 14): "It would seem preferable to distinguish the raphe ridge [defined above] from the raphe keel in which specialized supporting structures i.e., fibulae are present." The shape of the raphe fins will scarcely be seen with LM although their presence should be discernible in girdle view, especially when phase or interference contrast optics are used.

Common characters:

Intact frustules as well as single valves usually lying in broad girdle view.

Valves linear to lanceolate.

Valves and raphe straight and not sigmoid.

Raphe more or less ridged and raised above the general level of the valve.

Raphe without fibulae. Large helictoglossae.

Characters showing differences between genera: Shape of frustule in girdle view. Valves vaulted or ridged. Shape of valve ridge in girdle view.

Size of the two parts of the valve face on each side of the raphe (equal, subequal, or unequal in area). The presence or absence of raphe fins.

1. Former Navicula—usually solitary Genus Ephemera Paddock 1988

Type: Ephemera planamembranacea (Hendey) Paddock. Monospecific genus.

Ephemera planamembranacea (Hendey) Paddock (Plate 64) Basionym: Navicula planamembranacea Hendey.

References: Hendey, 1964, p. 188, Text Fig. 8; Paddock, 1988, p. 86, Plate 31. Cells usually solitary. Valves highly vaulted and flattened in transapical plane. Raphe separating the valve face into two unequal parts. Central nodule slightly depressed, usually with four stronger and more widely spaced interstriae, reaching valve margin and producing "the impression of a pseudo-stauros" (Hendey, 1964, p. 188). Small raphe fins near valve ends and one on each side of the central nodule. Numerous small and rounded chloroplasts. Morphometric data: Pervalvar axis, 15-30 jum; apical axis, 66-90 fim; transapical axis, 8-10 ^m; transapical striae, 28-30 in 10 /xm. Distribution: North Atlantic Ocean.

Remarks: Due to the shape of the cell it is usually seen in girdle view where the central and terminal nodules of the raphe are more distinct than the delicate striae. Ephemera planamembranacea is more similar to "Tropidoneis" than to Navicula sensu stricto and is therefore dealt with under this group, although, in the past, it has never been referred to Tropidoneis. It is also similar to Stauropsis but differs from this genus in chloroplast number and shape and by usually being solitary.

PLATE 64 Ephemera planamembranacea: cell in girdle view with four stronger, central interstriae, small paired raphe fins, and large helictoglossae. After Paddock (1988). Banquisia belgicae: cell in girdle view with raphe fins. Membraneis challengerii: cell in girdle view with reinforced striae, raphe fins, and enlarged helictoglossae. Scale bars = 10 fj.m.

Banquisia belgicae

Membraneis challenged:

2. Former Tropidoneis spp.—usually in ribbons.

2a. Valves lying in girdle view vaulted to a high ridge.

Genus Banquisia Paddock 1988

Type: Banquisia belgicae (Van Heurck) Paddock.

Monospecific genus.

Banquisia belgicae (Van Heurck) Paddock (Plate 64, Table 72) Basionym: Amphiprora belgicae Van Heurck.

Synonym: Tropidoneis belgicae (Van Heurck) Heiden (in Heiden 8c Kolbe, 1928).

References: Van Heurck, 1909, p. 14, Plate 1, Figs. 11 and 15; Heiden 8c Kolbe, 1928, p. 655, Plate 4, Figs. 98 and 99; Paddock, 1988, p. 79, Plate 28; Paddock, 1990, p. 153, Fig. 10. Raphe biarcuate in girdle view: a depression at central nodule and another at about one-third of the raphe's length from the valve apices. Poles steep. A pair of raphe fins at or near lowest point in raphe outline. Valves narrow with equal parts of valve face. Punctate transverse valve striae clearly seen with LM. Interstriae in the middle of the valve slightly broader than the others.

Genus Membraneis Paddock 1988

Type: Membraneis challenged (Grunow, in Cleve 8c Gronow) Paddock.

Generic characters:

Cells more or less lens shaped in girdle view. Valves with unequal or subequal parts of valve face.

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

TABLE 72 Morphometric Data of Banquisia, Membraneis, Manguinea, and Plagiotropis spp. (after Paddock, 1988)

Species Apical axis (p.m) Transverse striae in 10 pm

TABLE 72 Morphometric Data of Banquisia, Membraneis, Manguinea, and Plagiotropis spp. (after Paddock, 1988)

Species Apical axis (p.m) Transverse striae in 10 pm

Banguisia belgicae

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