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Abstract
Cyclostephanos kyushuensis sp. nov. is described from a Pliocene diatomite in Kyushu, southwestern Japan. The species is characterized by different morphologies of its convex and concave valves. The convex valves have shorter costae; those bearing a rimoportula or fultoportula extend to valve edge while the others do not. The concave valves have longer costae, all of which do not extend to valve edge in either external or internal views. Though the costae of some Cyclostephanos do not extend to valve edge, the presence of costae that do in convex valves and costae that do not in concave valves of the same frustule is unique for the genus. The new species is compared to similar species of Cyclostephanos: C fenestratus Theriot & Kociolek, C. delicatus (Genkal) Kling & Håkansson, C. dubius (Fricke) Round and C. numataensis Tanaka & Nagumo.