la. Cells solitary or in ribbons 2
lb. Cells solitary L. duckerae von Stosch
2a. Marginal ridge only partly perforated 3
2b. Marginal ridge with clear pattern of perforation
L. undulatum Ehrenberg
3a. Valves triangular, perforation of marginal ridge without a clear pattern and restricted to its advalvar half
3b. Valves more or less irregularly biangular, triangular, or quadrangular, marginal ridge sparsely or not perforated L. variabile Takano
L. duckerae—warm water region, probably benthic (von Stosch, 1987). L. intricatum and L. undulatum—warm water region to temperate. L. variabile—recorded in the literature only from Japanese coastal waters, but probably also present in Gulf of Naples (G. Hasle, personal observations).
How to identify: As for the other genera of this family a combination of girdle and valve view examinations may be needed. The distinction between L. undulatum and L. intricatum manifest in the perforation of the marginal ridge may be seen in water mounts but more clearly in a medium of a high refractive index.
Remarks: Lithodesmium undulatum and L. intricatum differ in the shape of the valve mantle and the marginal ridge. The valve mantle of L. intricatum has a widening in its transition to the girdle and the marginal ridge is inclined outwards or inwards. The valve mantle of L. undulatum is at a right angle to the valvar plane, and the marginal ridge is in the same plane as the mantle and the girdle. The sides of L. undulatum valves are undulated, whereas those of L. intricatum are straight or slightly concave. Lithodesmium duckerae is similar to L. intricatum but has higher elevations; the depressions across the corners are on an average deeper, the valve face areolation is much coarser, and there are siliceous spines in the valve center. The taxonomic position of L. variabile is dubious. Von Stosch (1987) suggested a similarity to Lithodesmioides based on the presence of a nonperforated and low marginal ridge. It has, however, depressions across the valve corners, and our own observations indicate the presence of some perforation of the marginal ridge, and, as also concluded by von Stosch (1987), its correct position therefore seems to be in Lithodesmium.
Genus Helicotheca Ricard 1987 Synonym: Streptotheca Shrubsole 1890.
Type: Helicotheca tamesis (Shrubsole) Ricard; see Ricard (1987, p.75). Monospecific genus.
Helicotheca tamesis (Shrubsole) Ricard (Plate 48) Basionym: Streptotheca tamesis Shrubsole.
H Basionym: Triceratium intricatum T. West.
References: Shrubsole, 1890; Hustedt, 1930, p. 778, Fig. 455; Cupp, 1943, p. 147, Fig. 106; Hendey, 1964, p. 113, Plate 7, Fig. 2; Drebes, 1974, p. 98, Fig. 80c; Hasle, 1975, p. 126, Figs. 131-141; von Stosch, 1977, p. 134, Figs. 78-84.
Girdle view: Chains ribbon shaped and separable. Chains and cells with a torsion in relation to pervalvar axis. Cells in broad girdle view rectangular apart from the torsion. Chloroplasts numerous and broadly oval with a slight constriction, each with a pyrenoid. Valve view: Valves linear oblong and slightly inflated in the middle. Each valve with a subcentral bilabiate process, the external part a low ring, and the internal part with a long stalk. Valve structure not resolvable with LM.
Morphometric data: Pervalvar axis, 56-120 /xm, apical axis, 26-160 /jlin;
transapical axis, 9-11 fim.
Distribution: Warm water region to temperate.
How to identify: The species may be identified in chains in water mounts and as single cleaned valves mounted in a medium of a high refractive index by the valve outline and shape and position of the process.
Genus Neostreptotheca von Stosch 1977 emend, von Stosch 1987 (Plate 48, Table 61)
Type: Neostreptotheca subindica von Stosch.
References: von Stosch, 1977, p. 134, Figs. 85-94; von Stosch, 1987, p. 78, Figs. 209-234.
Cells in straight separable ribbons.
Intercellular spaces missing.
Cells in broad girdle view shaped like a parallelogram.
Cells in valve view linear oblong and slightly inflated at the apices and in the middle.
TABLE 61 Morphometric Data of Neostreptotheca spp.
Apical axis Transapical axis (p.m) (f.m)
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