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Abstract
Biddulphia rhombus var. atlantica was erected by Frenguelli from waters of the South Atlantic Ocean. A revision of its syntype series was carried out with light microscopy, a lectotype was chosen and compared with the lectotype of Odontella rhomboides (= Zygoceros rhombus), and with the lectotype of Odontella rhombus (= Denticella rhombus), designated in this paper. As there is no unmounted material available in the Frenguelli Collection, a SEM study was carried out on material collected in Buenos Aires Province coastal waters, Argentina. Differences in pattern of striation and distribution of labiate processes were established as the striking features to distinguish Frenguelli's taxon from the other two species.
Based on the analysis of the fine structure the placement of Frenguelli's var. atlantica in the genus Odontella according to the modern concept of generic limits within the biddulphioid diatoms was confirmed and the taxon raised to species rank as Odontella atlantica, comb, et stat. nov.