Difflugia bicornis Penard, 1890
Diagnosis: Shell transparent, ovoid or spherical, slightly compressed. Typically with two spikes, but sometimes with only one, located aborally. Contour uneven, with the test covered by small and medium-sized grains of sand, diatoms, and flagellate cysts. The spikes are covered with fine grains of sand. The aperture is round.
Dimensions: Penard (1890): Shell length 50-60 µm (including horns). My measurements: Length 115-143 µm (n=13; 10 with two horns and 3 with one); L/W ratio 1.4-1.6.
Ecology, Freshwater, first described from Wiesbaden, Germany.
Remarks, The validity of this species is questionable. It is possible that Difflugia bicornis is identical to small specimens of Cylindrifflugia elegans. However, during my cultivation of C. elegans, I never observed specimens with two horns. In his seminal 1902 work, Penard no longer recognizes D. bicornis as a separate species, instead noting that he had originally referred to two-horned specimens of C. elegans as D. bicornis in 1890. In the Penard Collection, there is a slide containing a shell approximately 120 µm in length, labeled by Penard as D. bicornis (see photo at the end of this page).
