Pentagonia shablensis  Todorov & Golemansky, 1998

Diagnosis: Shell transparent, ovoid elongate, swollen in the mid-region but tapering at both extremities, to give a rounded protuberance aborally and gradually near the aperture, to give a slightly pronounced neck. Shell with three or four rounded longitudinal ribs which give a triangular or quadrangular shape of the shell in transverse section. shell composed mainly of small to medium flattish pieces of quartz so arranged that give a relatively smooth surface. The aperture is circular and surrounded by mainly small particles of quartz.

Measurements. Length 81-100; Width 52-65; Aperture 20-28; Length of collar 3-4 (n=23). My measurements: 78-82 µm (n-6).

Geographical distribution: Netherlands, Bulgaria.

Remarks: This species is similar to P. maroccana Gauthier-Lievre & Thomas (1958), but may be distinguished from it by the absence of a clear pronounced aboral spine or horn and by the smaller and not curved collar. Furthermore, P. shablensis is distinct in having a larger ovoid shell with a more open aperture.
In 2021, I collected specimens from the Gooimeer, a large lake in the Netherlands. These specimens had an irregular outline in top view, ranging from round, triangular to square (see below). Originally, I considered these specimens as Difflugia mammillaris . The form of Pentagonia shablensis in particular raises the question of whether it is a junior synonym of D. mammillaris and to what extent it is justified to maintain Pentagonia as an independent taxon.