Basionym: Rhizosolenia minima Levander.
Synonym: Monoceros isthmiiformis Van Goor.
References: Levander, 1904, p. 115, Figs. 7 and 8; Van Goor, 1924, p. 303, Fig. 3; Hustedt, 1930, p. 598, Fig. 343; Marino et al., 1991, p. 318, Figs. 1-9. Cells isovalvate. One seta per valve; the two setae running in opposite directions. Resting spores ellipsoidal; primary valve with scattered protuberances, knobs, and sometimes spines; secondary valve with regularly arranged numerous small knobs, a strong stud-like central protuberance, a wide flange, and a marginal collar.
Chaetoceros throndsenii (Marino, Montresor, 8c Zingone) Marino, Montresor, 8c Zingone var. throndsenii (Plate 47, Table 56) Basionym: Miraltia throndsenii Marino, Montresor, 8c Zingone. References: Marino et al., 1987; Marino et al., 1991. Cells heterovalvate. One seta per valve; the two setae of a cell running in same direction. Primary valve of resting spores strongly convex with numerous small protuberances; secondary valve more flattened, with one or two central stud like protuberances, a wide flange and a marginal collar.
Chaetoceros throndsenii var. trisetosus Zingone (in Marino et al., 1991) Reference: Marino et al., 1991, p. 319 Differs from the nominate variety by having a third seta.
C. minimus—brackish water: northern part of the Baltic Sea, Dutch inshore waters, Tyrrhenian brackish water lagoons. C. throndsenii—Gulf of Naples. Remarks: The small size and the weak silicification make the species difficult to recognize. Both species may appear in cell concentrations amounting to millions per liter (Marino et al., 1987). Whereas C. minimus is a typical
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