Triquetrorhabdulus rugosus Bramlette & Wilcoxon, 1967
Elongate nannolith with three nonbirefringent blades in variable width (having vertical c-axis - vertical to the natural settling of nannolith in the plan view). The side view displays a strong birefringent with a tangential c-axis. The two of the blades are often in the same plane. One end is often more pointed than the other. The surface of the widest lateral blade [dentate blade] is often ornamented with transverse striations. The narrowest blade [=median blade] may be straight or gently curved.
The species of Pennalithus were identified based on the shape of the median blade [straight or curved], size and shape of the lateral blades, presence or absence of ornamentations on the lateral blades and arching of lateral blades.
Pennalithus differs from Velonalithus and Triquetrorhabdulus by having a vertical c-axis in the plan view, whereas the latter genera have tangential and axial c-axes, respectively.
At their side views, Pennalithus and Velonalithus are very difficult to distinguish.