The original description by Skvortzow (1924) is very short and he placed this flagellate in Menoidium. Authors after him like Huber-Pestalozzi (1955) and Angeler et al. (2002) doubt if the short description is sufficient to describe a species. Furthermore, Huber-Pestalozzi is of the opinion that Menoidium tremulum actually belongs to the genus Rhabdomonas, because the body is not flattened, but is rotund. Skvortzow himself indicates the cells to be cylindrical. Finally, Huber-Pestalozzi tranferred into the genus Rhabdomonas and renamed the species in Rhabdomonas tremula. 

I found Rhabdomonas tremula in the mud of the Purren pond. At first sight it looks similar to Rhabdomonas costata, but it is much smaller. In addition, the large paramylon grains present are all located in the anterior quarter of the cell (as drawn by Svortzow, s. above). The longitudinal furrows of the pellicle only very weakly run in a clockwise spiral to the posterior end. My specimen is still slightly smaller than Skvortzow indicates, but still within usual variations.