Phylum: Charophyta   Family: Desmidiaceae

Euastrum crassum var. microcephalum W. Krieger 1937

Similar to the nominate variety but with a less pronounced pore surface and with four large and deep scrobiculae. Krieger, W. (1937: 512, pl. 65: 6) notes: Slightly slimmer than the type, more than twice as long broad. Apex part of the semcell smaller, evenly distributed very long basal part.
Krieger’s cell dimensions: L: 100-230 µm; B: 45—80 µm; Is: 16-30 µm; Apex: 25-62.5 µm. (That’s a huge size-range just for the apex?)

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Krieger, W. (1937) Die Desmidiaceen Europas mit Berücksichtigung der aussereuropäischen Arten.
Ruzicka, J. (1981) Die Desmidiaceen Mitteleuropas, Volume 1, Part 2.