| Feature/species | P. polonica (Witak & Lange-Bertalot) E. Morales & M.B. Edlund | P. occulta sp. nov. | P. oliveraiana Grana, E. Morales, Maidana & Ector | P. subsalina (Hustedt) E. Morales | P. zolitschkae M.L. Garcia, S. Bustos, Maidana & E. Morales |
| Valve dimensions (µm) | L: 8-23 W: 3-4 | L: 6.7-35.6 W: 3.3-3.8 | L: 19.0-39.5 W: 3.5-5.5 | L: 10-36 W: 4.0-5.3 | L: 8.5-28.0 W: 3.5-5.0 |
| Stria density (in 10 µm) | 13-15 | 14-16 | 11-17 | 13-14 | 11-14 |
| Valve outline; axial area; virgae | Broadly elliptic, rarely clavate, broadly rounded apices, larger specimens slightly constricted in the middle; lanceolate, at same level as virgae in both external and internal views; wider than striae | Lanceolate with subrostrate, squarish, broadly rounded apices; lanceolate, faintly depressed with respect to virgae in external and internal view; wider than striae | Lanceolate with subcapitate to cuneate ends; broadly lanceolate, at same level as virgae in both external and internal views; wider than striae | Lanceolate with parallel sides and subrostrate apices; linear to narrowly lanceolate, faintly depressed with respect to virgae in external and internal view; wider than striae | Lanceolate with subrostrate apices, smaller valves biconvex with cuneate, pointy ends; broadly lanceolate, at the same level as virgae in external view, slightly depressed in internal view; much wider than striae |
| Areolae; volae; striae | Wide, elliptic; well-developed, originating from longer areolar axis, and forming a tight mesh in internal view; rarely formed by more than 3 areolae, larger on valve | Narrow, round to elliptic; developed, originating from the inner areolar periphery, forming a tight mesh-like structure visible internally; with up to 4 areolae on valve face and up to 2 on valve mantle, usually larger near valve face/mantle transition | Narrow, elliptic to trapezoid; developed, originating from longer axis of areolae; with up to 3 areolae, larger on valve face | Narrow, round to elliptic; poorly developed, originating from inner areolar periphery and projecting inwards; with up to 5 areolae, decreasing in size towards the axial area, a single larger areolae present on valve mantle | Narrow, elliptic to 8-shaped, trapezoid on valve mantle; developed and forming a mesh-like pattern in inner view; with 2 areolae of about the same size on face and mantle |
| Spines; stipules; flaps | Hollow, with elliptic base, narrower than basal vimen, flattened body with very openly concave sides, spatulate tips with somewhat straight bifurcate projections; present but poorly developed; poorly-developed on valve face and mantle | Solid, with elliptic to rectangular base, as wide as basal vimen, cylindrical body with open concave sides, with spatulate tips that bear two lateral projections and serrate borders; incipient, subtending a circular depression on spine body; poorly developed on valve face, well-developed on valve mantle, spatulate | Solid, round to elliptic base, as wide as basal vimen, flattened body and tips with an overall inverted trumpet shape, tips bifurcate once or twice; absent; absent | Solid, round to elliptic base, as wide as basal vimen, flattened body with somewhat straight but flaring sides and spatulate tip bearing two small lateral projections with serrate borders, with an overall ice cream cone shape; incipient; poorly developed | Hollow, with round to elliptic base, narrower than basal vimen, with flattened body and typically T-shaped tips; well-developed and lobed; developed on valve mantle, lobed |
| Apical pore fields; mantle abvalvar blisters | Somewhat developed, with a few, large, round pores, sunken into an irregular depression at the valve apex, internally opening into a small roundish depression; well-developed, apparently not present at apices | Developed, externally covered by contorted flaps in external view, internally opening in a single ca. elliptic depression, revealing several rows of round poroids; well-developed, also present at valve apices | Somewhat developed, externally sunken into an elliptic depression at the valve apex, internally opening into a ca. elliptic depression with an elevated central area, and showing several round poroids; developed but absent from apices | Somewhat developed, sitting on a step-like transition between valve face and mantle, externally large roundish pores lie in an irregular depression, internally, the narrow, round pores open into a single roundish depression; developed but present at apices | Small, externally reduced to large pores with cavernous appearance, internally, small round pores lie in a roundish depression; developed, absent from apices |
| References | Morales & Edlund (2003) | This study | Grana et al. (2018) | Cejudo-Figueiras et al. (2011) | Garcia et al. (2021) |