Triquetrorhabdulus rugosus Bramlette & Wilcoxon, 1967
An elongated nannofossil with two unequal blades in the same plane and a median ridge. One end of the blades is often more pointed than the other. The surface of the wider blade (dentate blade) is frequently ornamented with transverse striations. The median ridge may be straight or curved.
Optical Properties: In the plan view, when the ridge is aligned with the microscope axis, Pennalithus is in constant extinction, with the optic axis perpendicular to the microscope stage in this orientation (Plate 19, Figs. 1-3, 5-7 and Plate 20, Figs. 1, 3, 5-7 in Varol, 2025). Pennalithus displays high birefringence in the side view, parallel extinction (extinction angle is 0°), and exhibits length-slow (+) elongation (Plate 19, Figs. 9-11, 13-15 and Plate 20, Figs. 9-16 in Varol, 2025).
Pennalithus differs from Orthorhabdus by having parallel (straight) extinction, whereas the latter has inclined (oblique) extinction in its side view. Moreover, both ends of Pennalithus are pointed, whereas Orthorhabdus has one end pointed and the other truncated.
Varol, O. 2025a. A practical guide to optical studies of calcareous nannofossils. Grzybowski Foundation Special Publication. 29: 1-222