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" Mentioned in the text for comparison. ' —, No data. ' ?, Data uncertain.

" Mentioned in the text for comparison. ' —, No data. ' ?, Data uncertain.

Thalassiosira gravida, especially since both have scattered strutted processes on the valve face. The resting spore valve is distinguished from T. gravida (see below) by having only one or two strutted processes in the valve center, one or sometimes two definite marginal rows of strutted processes, coarser areolae, and occasionally occluded processes. The vaulted secondary valve of the northern variety is more coarsely silicified than the flattened primary valve and may be identical with Coscinodiscus subglobosus Cleve 8c Grunow in Grunow (1884, p. 84, Plate 4, Figs. 19 and 20), formerly interpreted as resting spore of T. gravida. In the same way the resting spore of the southern variety may have been described as Coscinodiscus (subglobosus var.?) antarcticus by Grunow (1884, p. 84, Plate 4, Fig. 23). The fact that the resting spores may have been described before the vegetative cells as well as the fact that Cleve (1896a, p. 13) confused T. antarctica var. borealis and T. gravida creates nomenclature complications which require further consideration.

Thalassiosira baltica (Grunow) Ostenfeld (Plate 8, Table 10) Basionym: Coscinodiscus polyacanthus var. baltica Grunow in Cleve 8c Grunow.

References: Cleve &c Grunow, 1880, p. 112; Ostenfeld, 1901a, p. 290, Fig. 3; Hustedt, 1930, p. 328, Fig. 164; Hasle, 1978b, p. 266, Figs. 5-11;

Makarova, 1988, p. 63, Plate 33, Figs. 1-13; Hasle 8c Syvertsen, 1990b, p. 290, Figs. 28-30.

Girdle view: Cells almost rectangular; pervalvar axis about one-third to one-half of cell diameter; valve face flat; mantle low and slightly slanting. Connecting thread about twice as long as pervalvar axis. Valve view: Areolation fasciculate. Two to nine central strutted processes; two rings of marginal strutted processes situated on valve mantle together with three or four labiate processes.

Distinctive features: Fasciculation and number of labiate processes.

Thalassiosira gravida Cleve (Plate 8, Table 10) References: Cleve, 1896a, p. 12, Plate 2, Figs. 14-16; Fryxell, 1975, pp. 133-138; Syvertsen, 1977; Fryxell, 1989, Figs. 19-21. Girdle view: Cells rectangular; valve face flat; mantle low and slightly sloping. Connecting thread thick, especially close to valve surface. Valve view: Hexagonal areolae in radial rows. Numerous central strutted processes and scattered strutted processes on valve face and mantle; one large marginal labiate process.

Distinctive features: Connecting thread; hexagonal areolae; scattered strutted processes. Remarks: It should also be noted that T. gravida has not been found to form resting spores (Syvertsen, 1977), whereas resting spores corresponding in structure to C. subglobosus, regarded by Cleve (1896a) as endocysts of T. gravida, have been found as resting spores of T. antarctica (Syvertsen, 1979). Thalassiosira gravida may appear in mucilage colonies in the Antarctic (Fryxell, 1989).

Thalassiosira hyalina (Grunow) Gran (Plate 8, Table 10) Basionym: Coscinodiscus hyalinus Grunow in Cleve 8c Grunow. References: Cleve 8c Grunow, 1880, p. 113, Plate 7, Fig. 128; Gran, 1897a, p. 16, Plate 1, Figs. 17 and 18; Hustedt, 1930, p. 325, Fig. 159; Hendey, 1964, p. 86, Plate 1, Fig. 6. Girdle view: Pervalvar axis about one-third of cell diameter; valve face flat or slightly convex; mantle low and rounded. Connecting thread thick. Valve view: Valve face with radial areolae rows or ribs; mantle always areolated. Two to 15 central strutted processes; one marginal ring of strutted processes with conspicuous external tubes; labiate process taking the place of a marginal strutted process. Distinctive features: Low cells; one ring of marginal processes. Remarks: The well-areolated valves belong most likely to resting spores (or resting cells) which occur in groups of two with no or very short connecting thread (E. Syvertsen, unpublished observations). The Arctic Thalassiosira constricta Gaarder, the Antarctic T. karenae Semina, as well as T. gerloffii

Rivera from South American Antarctic and Subantarctic waters have many central strutted processes and one marginal ring. In contrast to T. hyalina, T. constricta has resting spores drastically different from the vegetative cells, has higher cells, the marginal strutted processes are wider apart, and the labiate process is midway between two strutted processes (Heimdal, 1971). Except for the distribution and lack of observations of resting spores, Thalassiosira karenae is almost identical with T. constricta (Semina, 1981a). Tbaiassiosira gerloffii is generally coarsely silicified with a fairly high valve mantle and areolation more similar to T. angulata and T. pacifica but with marginal processes closer together (G. Hasle, unpublished observations; Rivera, 1981).

Thalassiosira rotula Meunier (Plate 8, Table 10)

References: Meunier, 1910, p. 264, Plate 29, Figs. 67-70; Hustedt, 1930, p. 326, Fig. 328; Cupp, 1943, p. 49, Fig. 12; Fryxell, 1975, pp. 95-100; Syvertsen, 1977; Takano, 1990, pp. 226-227. Girdle view: Cells flattened and discoid. The band (copula) next to the valvocopula sometimes broadened, septate ("unevenly thickened"); otherwise as T. gravida.

Valve view: Valve face with radial ribs and few tangential areola walls (i.e., poorly developed areolae) except on valve mantle; otherwise as T. gravida.

Distinctive features: Structure of valve face and the band next to valvocopula.

Remarks: Experiments indicated that the special copula was formed by nutrient deficiency. The difference between T. rotula and T. gravida in valve structure seemed to be temperature dependent, thus indicating a conspecificity or close relationship between the two taxa (Syvertsen, 1977).

Thalassiosira turnida (Janisch) Hasle in Hasle et al. (Table 10) Basionym: Coscinodiscus tumidus Janisch in A. Schmidt. References: Schmidt, 1878, Plate 59, Figs. 38 and 39; Hasle et al., 1971; Johansen 8c Fryxell, 1985, p. 176, Figs. 28-32; Fryxell, 1989, Fig. 18. Girdle view: Cells rectangular; valve face flat with slight, central depression or lightly convex; mantle rounded. Several connecting threads twinned to form one thick thread.

Valve view: Areolation varying from linear to eccentric to fasciculate. Numerous strutted processes in valve center in an irregular ring, sometimes with processes also inside the ring; strutted processes scattered on valve face; one regular marginal ring of strutted processes including three to nine labiate processes.

Distinctive features: Arrangement of central processes; number of labiate processes.

Remarks: Thalassiosira tumida has been regarded as an extremely variable species. Fryxell et al. (1986a) hypothesized the existence of two varieties, and Fryxell (1988) found a certain relationship between temperature and variation in areola pattern.

Distribution:

T. antarctica var. antarctica, T. karenae, and T. tumida—southern cold water region.

T. antarctica var. borealis, T. constricta, and T. hyalina—northern cold water region to temperate.

T. gerloffii—Chilean coastal waters, southern Atlantic Ocean (Rivera, 1981, p. 80; Lange, 1985), southern cold water region (Johansen & Fryxell, 1985).

T. gravida and T. rotula—cosmopolitan if regarded as one species. Diatoms identified as T. gravida from the Antarctic may belong to a separate taxon.

T. baltica—brackish water (e.g., Baltic Sea and the Arctic).

2c. +/— Modified ring of subcentral strutted processes.

Thalassiosira anguste-lineata (A. Schmidt) G. Fryxell & Hasle (Plate 9, Table 11)

Basionym: Coscinodiscus anguste-lineatus A. Schmidt. Synonyms: Coscinodiscus polychordus Gran; Thalassiosira polychorda (Gran) J0rgensen; Coscinosira polychorda (Gran) Gran. References: Schmidt, 1878, Plate 59, Fig. 34; Gran, 1897b, p. 30, Plate 2, Fig. 33, Plate 4, Fig. 56; Jargensen, 1899, p. 15; Gran, 1900, p. 115; Hustedt, 1930, p. 317, Fig. 154; Cupp, 1943, p. 44, Fig. 7; Hendey, 1964, p. 89, Plate 1, Fig. 11; Hasle, 1972b; Fryxell & Hasle, 1977, p. 73, Figs. 22-34; Rivera, 1981, p. 45, Figs. 29-47; Makarova, 1988, p. 55, Plate 25, Figs. 10-13, Plate 26, Figs. 1-14; Takano, 1990, pp. 182-183. Girdle view: Cells rectangular; valve face flat or slightly curved; mantle rounded. Many connecting threads in groups at some distance from valve center.

Valve view: Areolation varying from linear to eccentric to fasciculate. Central strutted processes in arcs in a ring at some distance from valve center, each arc with one to nine processes. Strutted processes with conspicuous external tubes in one marginal ring including one labiate process.

Distinctive features: Arrangement of central processes, coarse marginal processes.

Remarks: The subcentral strutted processes of coarsely silicified T. anguste-lineata specimens are often difficult to recognize, and these diatoms may

T. kushirensis

T. anguste-lineata

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