Tetracyclus glans

Valves are elliptic-lanceolate with a large central inflation and rounded to pointed apices. Thick transapical costae are mostly primary and evenly spaced at 2-3 in 10 µm. The striae are faint compared to the costae and are slightly radiate, more so toward the valve margins. Areola density is 21-27 in 10 µm. The axial area is narrow and linear, sometimes with poorly defined margins. Copulae contain septa that extend approximately 1/4 - 1/3 of the length of the valve. The above images were taken from populations from the Pacific Northwest and did not have rimoportulae, but 1-2 have been found on valves from European populations (Williams 1987).

Tetracyclus glans, T. rupestris and T. emarginatus comprise the three extant species within the genus Tetracyclus, which includes a large number of extinct species (Williams, 1987). North American T. glans valves are smaller than those observed in European populations (Williams 1987).

Tetracyclus lacustris is a later synonym for T. glans. Although T. lacustris is commonly used, T. glans is the correct name, by priority. Ehrenberg (1838) described the taxon first (as Navicula? glans). Later, Ralfs (1843) described the same organism as T. lacustris. See Williams (1989) and Williams (1996) for detail.