I find Phacus pleuronectes frequently and regularly at some of my sampling sites. In my population, the specimens of Phacus pleuronectes were mostly under 50 µm in size, although specimens up to 100 µm have also been described in the literature. So the size seems to be quite variable.

I consider the dorsal keel and the short spine to be important identification features. The dorsal keel reach the mid-body (s. fig. 2 a) and the caudal spine which is always straight and slender. It bends to the right (seen from the flat ventral side) with an angle of almost exactly 45° in relation to the longitudinal axis of the body (s. figs 1 a and 1 b).

Differentiation from the similar species Phacus orbicularis is difficult. This species is almost as wide as it is long and is always said to be over 50 µm long. However, as the shape and length of Phacus pleuronectes is also variable, there may be overlaps. Kusel-Fetzmann (2002) discusses whether only specimens under 40 µm in length should be counted as Phacus pleuronectes. She also cites the delicate transverse stripes between the longitudinal stripes of the pellicle as a distinguishing feature. The specimens with transverse stripes should be classified as Phacus orbicularis and those without as Phacus pleuronectes. In my experience, however, this distinguishing feature is also variable. The transverse striation is found in many Phacus species, but is not present in all specimens or is too weakly developed to make it visible under the light microscope. Other authors have also not cited this feature as a distinguishing characteristic. I therefore consider the specimens shown below to be Phacus pleuronectes, although they are larger than 40 µm and also have a transverse striation (s. fig. 2 c).