Extremely well preserved specimens of the Carboniferous microproblematicum Saccamminopsis Sollas, 1921, from the Visean of Poland reveal a primary structure of the chamber wall, different from that hitherto suggested in hypothetical reconstructions. The calcareous wall is regularly perforated by fine pores which sometimes appear to be bifurcated, but no larger holes are present. The new observations, however, do not resolve the question of affiliation to algae or foraminifers. Similarity of general morphology and shell microstructure justifies a synonymization of Saccam-minopsis the Famennian genus Baculella Conil & Dreesen, 1985. Saccamminopsis is indicative of an extremely shallow marine sedimentary environment.□Algae, Carboniferous, Foraminifera, problematica, Saccamminopsis.