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' Mentioned in the text for comparison.

' Mentioned in the text for comparison.

chains sometimes embedded in mucilage forming large colonies. Long tubular (occluded) processes.

Valve view: Areolae in radial rows with slight fasciculation in larger specimens. Strutted processes on valve alternating, located in three rings, scattered strutted processes on valve face of larger specimens; external parts of strutted processes conspicuous but much shorter than those of occluded processes located in a ring between valve face and mantle. Distinctive features: High pervalvar axis; occluded processes. Remarks: Rivera (1981) put T. chilensis Krasske into synonomy with T. delicatula Ostenfeld.

Thalassiosira diporocyclus Hasle (Plate 6, Table 8) References: Hasle, 1972a, p. 113, Figs. 25-45; Rivera, 1981, p. 60, Figs. 108-128; Takano, 1990, pp. 196-197. Girdle view: Pervalvar axis of about the same size as cell diameter; central part of valve flat; mantle extremely curved. Cells in irregular mucilage colonies containing vast numbers of threads from the cells. Valve view: Areolation fasciculate. Two marginal rings of strutted processes lacking external tubes; the single labiate process in peripheral ring.

PLATE 6 Thalassiosira delicatula: (a) short chain; (b) valve with processes. From Gaarder (1951). Thalassiosira diporocyclus: (a) mucilage colony; (b) valve with processes. Thalassiosira eccentrica: (a) chain in girdle view; (b) valve with process pattern. Scale bars = 10 /un.

Distinctive features: In water mounts, colonies formed by numerous distinct threads. On cleaned mounted material the two rings of processes with the labiate process in the peripheral ring. Remarks: Thalassiosira fragilis G. Fryxell is similar to T. diporocyclus in valve morphology and colony formation but differs by having two labiate processes and the strutted processes, especially those in the inner ring, wider apart (Fryxell et al., 1984).

Thalassiosira eccentrica (Ehrenberg) Cleve (Plate 6, Table 8) Basionym: Coscinodiscus eccentricus Ehrenberg.

References: Ehrenberg, 1841a, p. 146; Ehrenberg, 1843, Plate 1/3, Fig. 20, Plate 3/7, Fig. 5; Cleve, 1904, p. 216; Hustedt, 1930, p. 388, Fig. 201; Cupp, 1943, p. 52, Fig. 14; Fryxell 8c Hasle, 1972, p. 300, Figs. 1-18; Rivera, 1981, p. 64, Figs. 129-140; Makarova, 1988, p. 48, Plates 20 and 21; Takano, 1990, pp. 198-199. Girdle view: Depending on cell diameter, pervalvar axis from one-sixth to longer than the length of the diameter. Valve face flat; mantle low and rounded. Connecting thread about twice the cell diameter. Valve view: Areolae in curved tangential rows (eccentric structure) with a tendency of fasciculation. Central strutted process adjacent to a central areola surrounded by a ring of seven areolae; scattered strutted processes on valve face; two marginal rings of strutted processes with short external tubes; one ring of pointed spines further away from valve margin. Distinctive features: Areola pattern of valve center; marginal spines. Remarks: Thalassiosira angulata and T. pacifica with eccentric structure lack the central areola and process arrangement, the marginal spines and the processes scattered on the valve face. Thalassiosira mettdiolana Hasle 8c Heimdal has the same process pattern as T. eccentrica but is usually more weakly silicified with smaller areolae arranged in sectors (Fryxell 8c Hasle, 1972).

Thalassiosira hendeyi Hasle 8c G. Fryxell (Table 8) Synonym: Coscinodiscus hustedtii Muller-Melchers. References: Muller-Melchers, 1953, p. 2, Plate 1, Figs. 2-5; Hasle 8c Fryxell, 1977b, p. 25, Figs. 35-45. Girdle view: Pervalvar axis somewhat shorter than cell diameter. Valvocopula coarsely structured. Low, wavy marginal ridge. Two labiate processes with long external tubes.

Valve view: Hexagonal areolae in straight tangential rows (lineatus structure). Small strutted processes in three marginal rings (not resolved with LM).

Distinctive features: External tubes of two labiate processes; lineatus type of areolation; marginal ridge.

Tbalassiosira simonsenii Hasle 8c G. Fryxell (Table 8) Reference: Hasle 8c Fryxell, 1977b, p. 23, Figs. 26-34. Girdle view: Cells discoid; mantle low and slanting. Valve view: Hexagonal areolae in straight tangential rows (lineatus structure). Central strutted process adjacent to a small areola. Ribbed margin. Small strutted processes in two alternating marginal rings (not readily visible with LM); two labiate processes with long external tubes; occluded processes.

Distinctive features: Lineatus type of areolation, ribbed margin, occluded processes. Distribution

T. delicatula and T. eccentrica—cosmopolitan, exclusive polar regions. T. diporocyclus, T. fragilis, T. hendeyi and T. simonsenii—warm-water region to temperate.

T. mendiolana—South Pacific Ocean only.

2a. One or a few (two to six) subcentral strutted processes.

Tbalassiosira curviseriata Takano (Plate 7, Table 9) References: Takano, 1981, p. 34, Figs. 26-38; Hallegraeff, 1984, p. 498, Fig. 8; Takano, 1990, pp. 192-193. Girdle view: Cells discoid; valve face with central concavity; mantle high and slanting. Connecting thread long. Cells also found embedded in mucilage to form large colonies. Several large disc-shaped chloroplasts. Valve view: Areolation radial; just discernible with LM (cleaned, embedded material). One or probably more often, two strutted processes adjacent to a more or less off centered annulus. Marginal strutted processes conspicuous being equipped with two opposing wings; labiate process close to a marginal strutted process. Distinctive features: Off-centered valve structure, winged marginal strutted processes.

Remarks: Tbalassiosira tealata Takano has one central process and marginal processes with longer wings than those of T. curviseriata (Takano, 1980).

Tbalassiosira dicbotomica (Kozlova) G. Fryxell 8c Hasle (Table 9) Basionym: Porosira dicbotomica Kozlova.

References: Kozlova, 1967, p. 56, Fig. 11; Fryxell 8c Hasle, 1983, p. 54, Figs. 2-14; Johansen 8c Fryxell, 1985, p. 161, Figs. 21-23 and 33-35. Girdle view: Valve face flat; mantle extremely low. Valve view: Areolae in radial; partly wavy rows; areolae walls thickened in pervalvar direction (SEM), visible with LM as irregular ribs. One to six central processes; labiate process midway between two marginal strutted processes, all with external tubes.

Distinctive features: Irregular ribs superimposed on the areolation although variable in shape and presence.

PLATE 7 Micrographs: single valves. Thalassiosira curviseriata: (a) areolae, process pattern, and characteristic outer parts of strutted processes, LM. Scale bar = 5 /tm; (b) as described in a, TEM. Scale bar = 1 ¿im; (c) external valve surface, SEM. Scale bar = 1 fim.

TABLE 9 Morphometric Data of Thalassiosira spp. Having One to Six Subcentral Strutted Processes

Species

Diameter (|tm)

Valve areolae in 10 pm

Marginal proc. in 10 |xm

T. curviseriata

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