Fig. 3

Latex cast of a Mississippian (Tournaisian/Visean) screwstone, NHMM MD 8154 (compare with Fig. 2d), from the bedload of the River Maas, the Netherlands. Note that the columnals are of similar height; it is difficult to judge if these are identical (homeomorphic) or show slight regular alternations of height (heteromorphic N1). Specimen coated with ammonium chloride. Scale bar represents 10 mm