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Petalomonas spinifera (Lackey, 1962) Lee and Patterson, 2000. Cell outline is irregular. Cells are about 29 microns long and 21 microns wide, flattened, and with three fine dorsal ridges, two fine ventral ridges and one distinct ridged ventral groove, which extends from the collar around the flagella canal. The cells have four hyaline protrusions: one is located on a wedge-shaped posterior end of the cell, two are in the lateral posterior part, and the last one is in the anterior left hand side or near the nucleus. The one emergent flagellum is slightly shorter than the cell and is anteriorly directed. The large reservoir is situated anteriorly in the right side of the cell and the nucleus is near the midline of the left side.
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Petalomonas cantuscygni Cann and Pennick, 1986. Cell rigid, flattened dorsoventrally, 6 dorsal keels, single ventral canal. Single flagellum inserted into a deep vacuole, no contractile vacuole. Marine in nature, but grows in marine and freshwater. Nucleus in middle of cell. Length 9-15 microns, width 8-8.5 microns
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Petalomonas excavata Skuja, 1939. Cells oval and a little flattened. Left side strongly convex, right side less convex. Cells 30-35 x 22-25 microns, 16-18 microns thick with a large median dorsal groove. Flagellum about cell length or slightly longer. Cell surface smooth, and cytoplasma hyaline filled with grains. Nucleus below the center or a little to the left side. Moves with a rather slow gliding.
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Petalomonas micra Norris, 1964. Cells spherical, slightly flattened, usually with a very short neck from which a single flagellum emerges. Flagellum two to three times longer than the diameter of the cell. Periplast firm, with no ridges or markings. Cell clear, containing numerous small spherical granules. Nucleus usually located in the posterior end of the call. Some cell contain irregular-shaped bodies, probably food vacuoles. Cells from 3.5 to 6 microns in diameter.
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Petalomonas phacoides Skuja, 1931. Cells oval or broadly egg-shaped, flattened, rigid, and 45-48 x 35-37 microns, thickness about 23 microns Ventral side is flat and the dorsal side is convex with 3 longitudinal keels. Flagellum about 1.5 times cell length. Vacuole is in right of the basis of the opening. There are a lot of roundish pellets (reserve substance).
