Aulacoseira iwakiensis sp. nov., a new elliptical Aulacoseira species, from an Early Miocene sediment, Japan

抄録

A unique Aulacoseira species with an elliptical valve face was found in Early Miocene sediment of the Shichiku Formation, Fukushima Prefecture, Japan. The valve face of the separation valve displays areolae covering the whole surface while the valve face of the linking valve has fewer areolae, mainly in the marginal area. Also, the mantle of the separation valve is curved while that of the linking valve is nearly straight, characteristics not observed before in any Aulacoseira species with an elliptical valve face. This paper describes the new taxon, Aulacoseira iwakiensis, and compares it to four similar taxa: Aulacoseira elliptica Tsoy, A. hachiyaensis H. Tanaka, A. ovata Usoltseva & Tsoy and Melosira distans var. ovata Iwahashi.