806. A NOTE ON AN INOCERAMID SPECIES (BIVALVIA) FROM THE LOWER CONIACIAN (CRETACEOUS) OF HOKKAIDO

Abstract

A large number of well preserved specimens from the limited thickness of fine-grained sandy siltstone in the lower part of the Coniacian sequence along the Pombets-Gono-sawa of the Mikasa [Ikushumbets] area, central Hokkaido, is regarded as representing a population of a single species which can be identified with Inoceramus rotundatus Fiege. We have examined statistically certain measurable characters on this sample (N=25) and described the extent of variation. The ratio of shell length to height is fairly constant, whereas that of breadth to height varies considerably. There is variation in the appearance and strength of major ribs and also in the distinctness of minor rings. Some specimens are thus fairly close to a form which has been called I. waltersdorfensis hannovrensis Heinz, but as there is gradation from the typical form of I rotundatus, they are included in the same species. I. rotundatus characterizes the lower part of the tripartite Coniacian in the sequence of the studied area. I. uwajimensis Yehara is occasionally found together with I. rotundatus, but it is more prolific in the middle part of the Coniacian sequence. The fossils associated with I. rotundatus include Didymotis akamatsui (Yehara), Forresteria (Reesideoceras) petrocoriensis (Coquand) and Harleites cf. H. harlei (Grossouvre).