It is suggested that Euastrum species characterized by cells lacking a sharp apical incision and with broadly rounded lobes, capped with series of conical wall granules, are centred on Euastrum mononcylum. From this species, the other representatives of the group may be considered as being derived by reductions or elaborations in their morphology. Greatest species diversity is found in the tropics and especially tropical Africa. Most of the taxa under discussion seem to be confined to relatively small geographic regions, possibly related to failures in sexual reproduction, dispersal barriers, and their recent evolution. The relatively high percentage of endemic taxa, not only in the Euastrum group under discussion, but also in other genera, renders the desmids an algal group particularly susceptible to losses in biodiversity.