北九州市立自然史博物館に寄贈されたイノセラムス（二枚貝類）

抄録

A huge specimen of Inoceramus from the Turonian of the Obira area, Hokkaido, donated to the Museum by Mr. Hiroharu Kokubu and another from the Turonian of the Hobetsu area, Hokkaido, by myself are described and illustrated under I. (I.) aff. iburiensis and I. (I.) iburiensis respectively.

On this occasion the specimen illustrated by Nagao & Matsumoto, 1939, pl. 31, fig. 1 is desgnated as the lectotype of I. (I.) iburiensis Nagao et Matsumoto and a specific diagnosis is given on the basis of this and other specimens. This species is somewhat similar to but can be distinguished from the nearly contemporary I. (I.) lamarcki Parkinson and the Coniacian species I. (I.) percostatus Müller. It is suggested that the group of I. (I.) lamarcki is represented in the Middle Turonian by similar but different subgroups of species between separate provinces of the North Temperate Realm and that they may have differentiated directly or indirectly from some unknown common ancestor(s) of the Early Turonian.