Some of the inoceramid specimens from the upper Turonian to lower middle Coniacian of the Obira and Ikushunbetsu areas, Hokkaido, are identical or well comparable with certain species, viz, Inoceramus (I.) Iusatiae Andert, I. (Volviceramus) koeneni Muller, I. (Cremnoceramus) deformis Meek, I. (Cr.) ernsti Heinz and I. (Cr.) lueckendorfensis Troger, all of which are cosmopolitan species that, however, have not been recorded in Japan. In this paper, I describe these specimens, subject them to some biometric analyses, and discuss their phylogeny and taxonomic position. Some specimens show intermediate, or transitional characters between Inoceramus and Cremnoceramus, Cremnoceramus and Cordiceramus, and Cordiceramus and Volviceramus, suggesting their phylogenetic relationships. The probable succession for the concerned taxa is preliminarily discussed and the taxonomic ranking of Inoceramus, Cremnoceramus and Volviceramus is also discussed. The above taxa are here regarded as subgenera of Inoceramus. Meanwhile, the widespread occurrence of these species in East Asia has application to international correlation and elucidation of paleobiogeography in the upper Turonian to lower middle Coniacian. In Part 1, I. (I.) lusatiae and I. (V.) koeneni are described and discussed.
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