Two ammonite species of the genus Sharpeiceras from the Cretaceous of Hokkaido : Studies of Cretaceous ammonites from Hokkaido and Sakhalin-LXXXI

Abstract

Sharpeiceras mexicanum (Bose, 1928) has been recently found in a mudstone unit of Hirotomi in the Monbetsu Valley, south-central Hokkaido. It possesses the adult body chamber, which has not been well shown in the hitherto described specimens of this species. Sharpeiceras kikuae Matsumoto and Kawashita, 1995 is redescribed in this paper, since it was established as an appendix to a stratigraphic paper written in Japanese and seems to have been little noticed. It came from a mudstone member in the lower part of the Middle Yezo Subgroup in the Oyubari area, central Hokkaido. It is somewhat similar to S. florencae Sapth but has a wider umbilicus, less crowded ribs on the septate whorls and more distinctly separated two lateral tubercles. In these respects it seems to resemble S. mocambiquense (Choffat), but the available material is insufficient for exact comparison. The two species described indicate an early Cenomanian age for the fossiliferous mudstone.