la. Areolae around valve central area elongated 2
lb. Areolae around valve central area not elongated 3
2a. Areolae on valve face conspicuously larger than those on valve mantle, marginal ridge entire and slotted, or fimbriate with ansulae
D. brightwellii13 (West) Grunow in Van Heurck
2b. Areolae close to central area especially large, the rest on valve face like those on valve mantle, the presence of ansulae variable
D. soln Grunow in Van Heurck
3a. Ansulae restricted to apical fourth of marginal ridge
D. buchananii von Stosch
3b. Ansulae along whole marginal ridge D. pernodii Schroder
D. brightwellii—cosmopolitan although not recorded from polar regions. D. buchananii—Gulf of Carpentaria, Australia, Gulf of Thailand (von Stosch, 1987).
D. pernodii—Papua, New Guinea, Gulf of Carpentaria, Townsville and Melville Bay, Australia (von Stosch, 1987). D. sol—warm water region. How to identify: The species may be discriminated by combined phase contrast examinations of whole frustules, preferably in a medium of a high refractive index (e.g., Pleurax) and of single valves cleaned of organic matter and mounted in a medium of a high refractive index. Remarks: Ditylum brightwellii and D. buchananii form resting spores (Hargraves, 1982; von Stosch, 1987). Ditylum buchananii and D. pernodii both have a very delicate ornamentation of the valve face. The triangular marginal ridge of D. buchananii has rounded corners, whereas the marginal ridges of D. pernodii and D. sol have subacute and narrow corners.
Genus Lithodesmioides von Stosch 1987 (Table 59) Type: Lithodesmioides polymorpha von Stosch. Reference: von Stosch, 1987, p. 46, Figs. 60-111.
13 Basionyms: Triceratium brightwellii T. West and Triceratium sol Grunow in Van Heurck, respectively.
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