van der Wiel (1982): Hyaline, irregularly cylindrical microfossils, measuring 19-380 (152) X 11-30 (22) µm, at one end provided with a pore of about 3 µm in diameter, always found broken off at the other end. These microfossils were found empty or filled with a smaller cylindrical structure of similar morphology. This inner structure has pores of ca 1 µm in diameter at both ends.
van Geel et al (1983b): Tube-shaped hyaline sheaths, straight or slightly curved, 60.0-167.5 X 15.5-32.5 µm. Walls 1.5-10 µm thick, the relatively thick ones showing a lamellate structure. The proximal end of the sheaths rounded, showing a depression with a ca. 3 µm wide pore, originally forming the connection with the globose heterocyst (which became detached and was not recognised as a characteristic "Type"). The distal end always open, originally surrounding the basal cells of a series of living algal cells (trichome). Some specimens show one or more constrictions of the sheath (Plate II, 146.c, d and g). Sheaths empty or still filled with an akinete in the proximal part. In some cases colonies of sheaths were observed. In the illustrated colony the original position of the heterocysts can be deduced from their impression in adjacent sheaths.
van Geel et al. 1989: Sheaths of the section Usselo measure up to 250 μm in length, with a diameter of 7-20 μm. The pore in the depression at the proximal end is 2-3 μm in diameter. Some sheaths contain a resting spore (akinete; see figs.146.d,h and i) with a c. 1 μm wide pore at both ends.
van der Wiel AM (1982) A palaecological study of a section from the foot of the Hazendonk (Zuid-Holland), based on the analysis of pollen, spores and macroscopic remains. Rev. Palaeobot. Palynol. 38:35-90.