Palustrimonas yorkeensis (Ruinen, 1938) Patterson and Simpson, 1996. Cell 8 -14 microns long, body not plastic and ovate to slender elliptical in profile. Anterior end may be somewhat pointed. The flagella insert sub-apically and are opposed. The anterior flagellum inserts into a short anteriorly directed pocket. The posterior flagellum inserts into a groove that extends for most of the length of the cell. The groove is often an elongated tear-shape with the point posterior-most. The anterior flagellum is approximately equal to the cell body in length and flails around the anterior end of the cell. The posterior flagellum is 2-2.5 times cell length and may be held fairly stiffly or beats in slow waves. The posterior flagellum generally appears thicker than the anterior flagellum. In some cells the posterior flagellum is missing. Swimming is often erratic and the cells tumble such that progress in one direction is quite slow. The cells do not glide. Many cells had large colourless inclusions in the posterior end of the cell suggesting that P. yorkeensis is predatory. In some cells the inclusions distended the cell sufficiently to obliterate the posterior portion of the groove. One cell was observed with an inclusion in the apical region which suggests that food may be ingested at the anterior end of the cell.
