(®gures 24i, 25n±p)

Description. Cell outline oval, about 18 Mm long and 11 Mm wide, somewhat dorso-ventrally ¯attened, rigid, with an anterior protrusion and a central nucleus. Cells have a deep ventral depression and are somewhat warty. Two ¯agella insert subapically into the depression; the anterior ¯agellum is about 0.5 times the cell length, and the posterior ¯agellum is slightly longer than the cell and inserts below the anterior ¯agellum. Moves by gliding with the anterior ¯agellum directed forwards and the posterior ¯agellum trailing. Three cells observed.

Remarks. The organisms observed are very similar to Heterochromonas opaca Skuja, 1948 in cell length and the position of nucleus, in having two ¯agella which diOEer in length, and having a ventral depression and the anterior protrusion. In the original description of Skuja (1948) the anterior ¯agellum is longer than the posterior ¯agellum. Heteronema opaca is similar to Diplonema papillatum (Porter, 1973) Triemer and Ott, 1990 in general appearance and cell length, but it can be distinguished because D. papillatum is ¯exible. Also, H. opaca resembles Abollifer prolabens Vùrs, 1992; A. prolabens has one ¯agellum, but sometimes the second shorter ¯agellum may appear before cell division (Vùrs, 1992a). Heterochromonas opaca resembles Cruzella marina (see de Faria et al., 1922), but can be distinguished because C. marina is ¯exible and has a long anterior ¯agellum. It is distinguished from other species of Heterochromonas by the anterior protrusion. This genus includes about 13 species; Heterochromonas bodoides Skuja, 1948, H. chlorophaga Skuja, 1956, H. cryptostigma Skuja, 1939, H. globosa Skuja, 1948, H. gotlandica Skuja, 1956, H. hovassei Fiatte and Joyon, 1965, H. nobilis Skuja, 1956, H. ovaca Skuja, 1948, H. polysticta Skuja, 1948, H. rotundata Skuja, 1956, H. spaerophora Skuja, 1956, H. vivipara (Ehrenberg, 1838) Pascher, 1912 and H. vulgaris (Cienkowsky, 1870) Pascher, 1912 (see Pascher, 1912; Skuja 1939, 1948, 1956).