1 a. Valves with one to three marginal rings of strutted processes 2
lb. Processes away from valve margin Minidiscus, p. 37
2a. Girdle with organic extrusions Planktoniella, p. 39
2b. Girdle with no such extrusions 3
3a. Valve wall alveolate Cyclotella, p. 33
3b. Valve.surface with loculate areolae or radial ribs; not alveolate 4
4a. Chain formation by external tubes of marginal strutted processes ... 5
4b. Chain formation by threads from strutted processes 6
5a. Central process present Detonula, p. 34
5b. No central strutted process Skeletonema, p. 43
6a. Strutted processes organized in a pattern on valve face 7
6b. Strutted processes scattered on valve face, no particular central processes or central processes rudimentary (EM) 8
7a. Adjacent cells in chains abutting Bacterosira, p. 31
7b. Cells in chains separated by shorter or longer distance, or cells solitary or embedded in mucilage Thalassiosira, p. 45
8a. Long occluded processes, central processes rudimentary or missing, valve structure mainly consisting of radial ribs Lauderia, p. 36
8b. Occluded processes absent, valve surface areolated Porosira, p. 41
Remarks: Strutted processes from which linking threads are extruded may be situated (1) exactly in valve center inside an areola (seldom) or an annulus, (2) next to a central areola (often) or an annulus, or (3) somewhere between valve center and margin, in some species in a more or less modified ring. Central and subcentral processes thus stand for "strutted processes through which interconnected threads instrumental in chain formation may be extruded."
Genus Bacterosira Gran 1900 Type: Bacterosira fragilis (Gran) Gran.
Correct name: Bacterosira bathyomphala (Cleve) Syvertsen 8c Hasle (vide Hasle 8c Syvertsen, 1993, p. 298). Monospecific genus.
Bacterosira bathyomphala (Cleve) Syvertsen 8c Hasle (Plate 1) Basionym: Coscinodiscus bathyomphalus Cleve. Synonyms: Lauderia fragilis Gran; Bacterosira fragilis (Gran) Gran. References: Cleve, 1883, p. 489, Plate 38, Fig. 81; Gran, 1897a, p. 18, Plate 1, Figs. 12-14; Gran, 1900, p. 114; Hustedt, 1930, p. 544, Fig. 310; Hendey, 1964, p. 141, Plate 7, Fig. 5; Hasle, 1973a, p. 27, Fig. 88; Hasle 8c Syvertsen, 1993, p. 298. Girdle view: Tight chains by abutting valve faces. Pervalvar axis usually longer than cell diameter. Apparent lens-shaped structure between cells. Cell wall weakly silicified.
Valve view: Cluster of central processes. One marginal ring of small strutted processes, one marginal labiate process. Radial ribs from valve center toward valve margin.
Resting spores: Semiendogenous, heterovalvate, primary valve with flattened valve face, secondary valve highly elevated in the center, first described as Coscinodiscus bathyomphalus (Cleve, 1883).
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