van Geel et al (1983b): Heterocysts ellipsoid to almond-shaped, 9.7-18.4 X 6.0-13.3 µm. Wall ca. 1 µm thick, hyaline. The narrow truncated end of the cell with a ca. 1 µm wide pore, originally forming the connection with the trichome. Often remains of the proximal part of the gelatinous sheath still in organic connection with the heterocyst.
Van Geel et al. 1989: Heterocysts almond-shaped, 9.8-12.0 X 5.6-8.1 μm, with a ca. 0.8 μm wide pore at the narrow end, which often bears an up to 2 μm long appendage (the sheath of the trichome, which did not become fossilized).
van Geel B, Hallewas DP, Pals JP (1983) A Late Holocene deposit under the Westfriese Zeedijknear Enkhuizen (Prov of Noord-Holland, The Netherlands): palaeoecological and archaeological aspects. Rev. Palaeobot. Palynol. 38:269-335.