(®gures 22j, 23i, j)
Description. Cell slightly oval or roundish, 12±32 Mm long, 10±27 Mm wide, dorsoventrally ¯attened, with thickened cortex. With a ventral median groove; cell indented anteriorly and posteriorly where groove meets margin. Subapically, the right margin of the groove forms a protrusion. With two ¯agella inserting under the protrusion; the anterior ¯agellum is about 0.5 times the length of the cell and the posterior ¯agellum is about 0.5 ±1.5 times the length of the cell. The nucleus is without nuclear caps, is located subapically in a median position, is rounded and is 5±13 Mm in diameter. Contained many food particles up to 5 Mm long. Commonly observed. Descriptions based on observations of 16 cells.
Remarks. This species was described from marine sites in tropical Australia, Fiji and England by Larsen and Patterson (1990) and Tong (1997b). Cell length was reported to be 8.5±20 Mm by previous authors. Generally, our observations are in accordance with those of Larsen and Patterson (1990) although our length range extends the previous range. Although the identities of most species within the genus are di cult to establish, the protrusion near the ¯agellar insertions makes this species easy to identify.