Abstract
Navicula densilineolata, this species is an optimum indicator for oligosaprobity at pH 7 or more alkaline pH and less than 100 mg/L chloride ion （Cl－）. However, there are many species resembling this species in size and shape of valve. For this reason, it is confused that the author who first announced this taxon proposes status change of this species few years later. In this paper, we researched the morphological variability and revealed the distinction points against 19 approximate species with the photographs of 230 valves of this species obtained in Lake Ikeda （Kagoshima Prefecture, Japan）. Finally, we created a description of this species.
