Two new Cyclostephanos species, C. nepalensis and C. pseudonepalensis (Bacillariophyta), from middle Pleistocene sediments of the Paleo-Kathmandu Lake, Nepal
Hayashi, Tatsuya; Tanimura, Yoshihiro
Two new Cyclostephanos species, C. nepalensis and C. pseudonepalensis, are described from middle Pleistocene sediments of the Paleo-Kathmandu Lake, Nepal. Valves of both species are characterized by (1) a concentric undulation, (2) fascicles composed of radial striae, (3) well-developed external costae, (4) weak internal costae, (5) internal domed cribra on both the valve face and the mantle area, (6) valve face fultoportulae internally with a central tube and two satellite pores, (7) mantle fultoportulae internally with a central tube and three satellite pores and (8) a single rimoportula located on a costa in the mantle area. In both species, valve size-related variations occur in the number of valve face areolae, mantle areolae, costae and mantle fultoportulae and in the internal shape of the rimoportula. Only C. nepalensis shows valve size-related variation also in the number of valve face fultoportulae. Additionally, both species show valve silicificationrelated variations, which occur as the annulus structure in the central part or as the groove and/or chamber-like structures in the marginal part. Cyclostephanos nepalensis and C. pseudonepalensis appear similar to C. fenestratus and C. numataensis, but they differ in the number of costae and in the appearance of valve face fultoportulae and the domed cribrum on the mantle area. Although C. nepalensis and C. pseudonepalensis are very similar to each other, they can be clearly distinguished by differences in the number of costae, valve face fultoportulae and mantle fultoportulae.