Abstract
In this paper three species of the ammonite genus Forbesiceras which occurred in the Lower Cenomanian of Hokkaido are described and figured. They are F. largilliertianum (d'Orbigny), F. beaumontianum (d'Orbigny) and a new species. The last one is well defined on the basis of at least eleven specimens from one and the same locality in Mikasa. It is allied to F. obtectum (Sharpe) and F. subobtectum (Stoliczka) but distinct in its own peculiar characters as described below. Through this study a clioscaphitoid shape of the adult shell of Forbesiceras has become distinct. The dimorphism which has been noticed by previous authors can be approved also by examples in Japan, but my observation is yet insufficient to draw up the true feature.
