Online ISSN : 2186-8565
Print ISSN : 0911-9310
ISSN-L : 0911-9310
34 巻
  • Tatsuya Hayashi
    2018 年 34 巻 p. 57-67
    発行日: 2018年
    公開日: 2019/02/01
    ジャーナル フリー

    A fossil nonmarine Actinocyclus species, A. krebsii sp. nov., is described from a lower to middle Miocene diatomite in the Chojabaru Formation, Nagasaki Prefecture, western Japan. The new species is characterized by having a valve face with concentric undulation, a shallow mantle, non- or vaguely fasciculate areolae, hyaline stripes extending from the valve face to the mantle, rimoportulae born from distal ends of the hyaline stripes, and a single pseudonodulus. Size-dependent variations are concentric undulation, the development of internal labia of rimoportulae, and the number of hyaline stripes and rimoportulae. Size-independent variations occur at the valve face/mantle junction: external expression of a hyaline area and internal expression of the pseudonodulus. Among nonmarine species of Actinocyclus, A. krebsii belongs to the non-fasciculate group that is characterized by having relatively few large and closely packed (low density) areolae on the valve face. Within this group, A. krebsii is most similar to A. venenosus because they both have long hyaline stripes that extend from the valve face to the mantle. Actinocyclus krebsii, however, has a higher density of areolae on the valve face and longer hyaline stripes than A. venenosus. Additionally, A. krebsii does not have the well-developed hyaline valve/mantle junction and spines that characterize A. venenosus.