Helicosphaera seminulum recta Haq, 1966
This helicolith features an asymmetrical blanket (Type II, Helicosphaerella recta type). The flange termination varies among species, either flush (e.g., Helicosphaerella compacta) or truncated (e.g., Helicosphaera truncata). The bar structure spanning the central opening of the proximal plate may be conjunct, as in Helicosphaerella philippinensis, or disjunct, as in Helicosphaerella robinsoniae. In some species, such as Helicosphaerella pacifica, the central opening is covered by a non-permanent mesh structure. The proximal plate lacks any distinguishable structure in some species, as seen in Helicosphaerella orientalis.
The orientation of the bar is a key distinguishing characteristic among Helicosphaerella species (Table 7 in Varol, 2025):
• Inverse bar: A diagonal bar with its anterior (front) end positioned at the side of the flange termination, as seen in Helicosphaerella obliqua (conjunct inverse). Disjunct inverse bars have not been observed in Helicosphaerella species.
• Transverse bar: A bar aligned with the short axis of the helicolith, as in Helicosphaerella truncata (conjunct transverse) and Helicosphaerella robinsoniae (disjunct transverse).
• Oblique bar: A diagonal bar with its posterior (rear) end positioned at the side of the flange termination, as seen in Helicosphaerella clarissima (disjunct oblique). Conjunct oblique bars have not been observed in Helicosphaerella species.
Optical Properties: The proximal plate and the flange remain in constant extinction, while the blanket exhibits first-order interference colours with length-fast (−) elongation.
Aubry, M. -P. & Bord, D. 2009. Reshuffling the cards in the photic zone at the Eocene/Oligocene boundary. In: Koeberl, C., Montanari, A. (eds), The Late Eocene Earth-Hothouse, Icehouse, and Impacts. Geological Society of America, Special Papers 452: 279-301.