The genus Monactinus is easily recognized because the marginal cells have only one horn-shaped process. Between the inner cells of Monactinus simplex large intercellular spaces are present (s. fig. 1 a). Sometimes the inner cells lacking so that the outer cells form a ring. The cell wall of the specimens in my population was granulated but noch covered with short spines as in the similar form Monactinus simplex var. echinulatum.
Fig. 1 a-b:Monactinus simplex. D = 85 µm. Two focal planes of a coenobium of 16 cells. There are intercellular spaces between the Y-shaped inner cells (a). The surface of this specimen is finely granulated (b). PY = pyrenoid. Obj. 100 X.