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' Valve face. k Valve mantle.

' Valve face. k Valve mantle.

Valve view: Areolae on valve face in straight tangential rows (lineatus structure), those on the mantle smaller and irregularly arranged; a central areola larger than the others. Small strutted processes in two to three marginal rings not revealed with LM; one ring of coarser processes further away from margin composed of one large labiate process and evenly spaced coarse, short occluded processes.

Distinctive features: Large labiate process and occluded processes; larger central areola.

3b. Zero to three subcentral strutted processes.

Thalassiosira guillardii Hasle (Plates 10 and 11, Table 12) Synonym: Cyclotella nana Guillard clone 7-15 in Guillard 8c Ryther. References: Guillard 8c Ryther, 1962, Plate 2, Figs. 2A and 2B; Hasle, 1978b, p. 274, Figs. 28-50; Makarova et al., 1979, p. 924, Plate 3, Figs. 8-10.

Girdle view: Cells almost rectangular; pervalvar axis from about one-third to as long as cell diameter. Colonies not observed.

Valve view: Areolation not resolved with LM; slightly raised siliceous ribs radiating from an annulus; less distinct in outer half of valve face (EM). Subcentral strutted processes located about one-third to one-half radius away from valve center; one marginal ring of regularly spaced strutted processes; one labiate process taking the place of a strutted process. Distinctive features: When compared with T. pseudonana, the location of the labiate process and an apparent lack of areolation.

PLATE 10 Thalassiosira guillardii: (a) cell in girdle view; (b) valve with two subcentral processes. Thalassiosira ientigitiosa: (a) cell in girdle view; (b) valve with process pattern. Thalassiosira leptopus: valve with pattern of marginal strutted and occluded (open circles) processes. Thalassiosira lineata: (a) cell in girdle view; (b) valve with process pattern. Scale bars = 10 fim.

PLATE 11 Micrographs: single valves. Thalassiosira pseudonana (a-c): (a) valve with proccss pattern, LM. Scale bar = 5 /u.m; (b) typical valve structure, TEM. Scale bar = 1 /xm; (c) external valve structure, SEM. Scale bar = 1 /un. Thalassiosira guillardii (d-f): (d) valve with proccss pattern, LM. Scale bar = 5 /xm; (e) valve process pattern, TEM. Scale bar = 1 /tm; (f) external valve surface, SEM. Scale bar = 1 /im.

Thalassiosira pseudonana Hasle 8c Heimdal (Plate 11, Table 12) Synonym: Cyclotella nana Guillard clone 3H in Guillard 8c Ryther. References: Guillard & Ryther, 1962, Plate 1, Fig. 1A; Hasle & Heimdal, 1970, p. 565, Figs. 27-38; Hasle, 1976b, p. 105, Figs. 11-16; Makarova et al., 1979, p. 923, Plate 2, Figs. 1-8, Plate 3, Figs. 1-7; Hasle, 1983a, Table 1; Simonsen, 1987, Plate 657, Figs. 20 and 21 (?), but probably not Figs. 13-19.

Girdle view: Cells rectangular; pervalvar axis shorter than or as long as cell diameter; valve face flat or slightly convex; mantle rounded. Colonies not observed.

Valve view: Valve structure not revealed with LM. Process pattern distinct in cleaned, mounted material. One marginal ring of strutted processes including one labiate process midway between two strutted ones. Distinctive features: When compared with Thalassiosira oceanica, the location and number of central strutted processes. Remarks: Thalassiosira pseudonana is an extremely variable species, and a precise description based on LM observations can hardly be given. The diatom here named T. pseudonana is identical with specimens in Hustedt's samples from Wümme (the type locality of Cyclotella nana) and with clonal cultures established from Wümme (Hasle, 1976b). The kind of valve structure, radial ribs, a more or less developed areolation as well as a central annulus, or a network of cross venation (as in T. weissflogii) are apparently dependent on the amount of silica available (Paasche, 1973). Chang 8c Steinberg (1989) "ordered T. pseudonana back to Cyclotella." Although T. pseudonana has characters not exactly consistent with the Thalassiosira type, it is hard to see that it is any closer to the Cyclotella type (e.g., Hákansson, 1989).

3c. Strutted processes on the whole valve face; no particular central or subcentral process.

Thalassiosira lentiginosa (Janisch) G. Fryxell (Plate 10, Table 13) Basionym: Coscinodiscus lentiginosus Janisch in A. Schmidt. References: Schmidt, 1878, Plate 58, Fig. 11; Fryxell, 1977, p. 100, Figs. 13 and 14; Johansen 8c Fryxell, 1985, p. 170, Figs. 7, 49, and 50. Girdle view: Cells narrowly rectangular; pervalvar axis about one-fourth diameter; valve flat; mantle low. Not observed in chains (Fryxell, 1977). Valve view: Areolation usually fasciculated. Strutted processes evenly scattered over valve face, resembling small areolae (not easily seen with LM); one marginal ring of inconspicuous strutted processes lacking external tubes; a large easily visible marginal labiate process. Distinctive features: Labiate process; fasciculate valve structure.

TABLE 13 Morphometric Data of Thalassiosira spp. with Strutted Processes on the Whole Valve Face and No Central or Subcentral Process

Species

Diameter (p.m)

Valve areolae or ribs in 10 (im

Marginal proc. in 10 |im

T. lentiginosa

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