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FIGURE 12 A light micrograph of a clonal culture of Thalassiosira punctigera shortly after auxospore formation, showing the size variation.

b. More than one marginal ring of strutted processes: T. delicatula, T. diporocyclus, T. eccentrica, T. fragilis, T. hendeyi, T. mendiolana, and T. simonsenii (Table 8).

2. Usually more than one central or subcentral strutted process.

a. One or a few (2-6) subcentral strutted processes: T. curviseriata, T. dichotomica, T. minima, and T. tealata (Table 9).

b. Cluster of central strutted processes: T. antarctica, T. baltica, T. constricta, T. gerloffii, T. gravida, T. hyalina, T. karenae, T. rotula, and T. túmida (Table 10).

c. Modified ring of subcentral strutted processes: T. anguste-lineata, T. australis, T. hyperborea, T. kushirensis,

T. mediterránea, and T. weissflogii (Table 11).

3. No or a variable number of strutted processes on valve face.

a. No strutted processes on valve face: T. leptopus (Table 12).

b. Zero to three subcentral strutted processes: T. guillardii and T. pseudonana (Table 12).

c. No particular central or subcentral strutted process: T. lentiginosa, T. lineata, and T. lineoides (Table 13).

B. Labiate process on valve face; external process tubes absent, one ring of marginal strutted processes.

1. One central or subcentral strutted process: T. bioculata,

T. gracilis, T. oestrupii, T. perpusilla, T. proschkinae, and T. rosulata (Table 14).

2. More than one central strutted process.

a. Cluster of central strutted processes: T. ritscheri (Table 15).

b. One or two rows of central/subcentral strutted processes:

T. confusa, T. frenguellii, T. poroseriata, and T. trifulta (Table 15).

c. Modified ring of subcentral strutted processes:

T. endoseriata and T. poro-irregulata (Table 15).

A. Labiate process(es) near valve mantle.

la. One central or subcentral strutted process. One marginal ring of strutted processes.

This is the most numerous Thalassiosira group containing species that are the most difficult to disf'nguish. Except for the few species with an invariable lineatus structure (arecin straight tangential rows), the areolation pattern varies with the size of tn* jell diameter from straight or tangential rows in the smaller specimens to distinct fasciculation in the larger ones, or from radial rows to fasciculation. Dependening on the degree of silicification the valve structure of one and the same species may vary from merely faint radial ribs to a fully developed areolation.

Thalassiosira allenii Takano (Plate 4, Table 7)

References: Takano, 1965, p. 4, Fig. 2, Plate 1, Figs. 9-11; Hasle, 1978a, p. 101, Figs. 100-128; Rivera, 1981, Figs. 432 and 433; Takano, 1990, pp. 180-181.

Girdle view: Cells quadrangular; valve face flat; valve mantle oblique. Connecting thread slightly longer than pervalvar axis. Valve view: Hexagonal areolae in straight or curved tangential rows, or in radial rows with a tendency of fasciculation; no distinct central areola; mantle areolae about half the size of those on valve face. Labiate process taking the place of a marginal strutted process; long, coarse external tubes of marginal processes.

Distinctive features: Similar to T. nordenskioeldii in girdle view but with lower mantle, marginal processes closer together, and much smaller mantle areolae.

Thalassiosira angulata (Gregory) Hasle (Plate 4, Table 7) Basionym: Orthosira angulata Gregory.

Synonym: Thalassiosira decipiens (Grunow) j0rgensen non Thalassiosira decipiens (Grunow) Jorgensen in Hasle 1979.

References: Gregory, 1857, p. 498, Plate 10, Figs. 43 and 43b; Jorgensen, 1905, p. 96 pro parte, Plate 5, Fig. 3a-e; Hustedt, 1930, p. 322, Fig. 158; Cupp, 1943, p. 48, Fig. 10, in both as Thalassiosira decipiens (Grunow) Jergensen; Hasle, 1978a, p. 93, Figs. 4 and 70-99; Rivera, 1981, Fig. 436; Makarova, 1988, p. 56, Plate 27, Figs. 1-4 and 7-10. Girdle view: Pervalvar axis usually shorter than the diameter. Valve face flat; mantle smoothly curved. Connecting thread distinctly longer than pervalvar axis. External tubes of marginal processes usually readily seen. Valve view: Hexagonal areolae in curved tangential rows (eccentric structure), sometimes in straight rows or in sectors; no distinct central areola. Marginal processes with long external tubes; wide apart. Distinctive features: Large labiate process with long external tube located close to a marginal strutted process; sharp distinction between valve mantle and valvocopula. Remarks: Thalassiosira decipiens (Grunow) j0rgensen differs morphologically from T. angulata by more closely spaced marginal processes and a lower valve mantle. Thalassiosira decipiens is unlike T. angulata, littoral more than planktonic, and is found in great inland seas, estuaries, bays, shallow coastal waters, and rivers influenced by the tide (Hasle, 1979).

PLATE 4 Thalassiosira allenii, T. angulata, T. bulbosa, and T. conferta. (a) Chains in girdle view; (b) valves with process pattern. Scale bars = 10 fim.

TABLE 7 Morphometric Data of Thalassiosira spp. with One Central or Subcentral Strutted Process and One Marginal Ring of Strutted Processes

Areolae in 10 |im

Diameter Valve

Spccics (lim) face Valve mantle Marginal

TABLE 7 Morphometric Data of Thalassiosira spp. with One Central or Subcentral Strutted Process and One Marginal Ring of Strutted Processes

Spccics (lim) face Valve mantle Marginal

T. aestivalis'

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