Abstract
Abstract. In this paper Inoceramus kamuy n. sp. is established on a fairly large number of specimens from Hokkaido. It ranges throughout the lower Turonian strata and is certainly useful for correlation. In its juvenile growth stages, I. kamuy closely resembles I. nodai of late Cenomanian age, but differs in later growth stages in that its valve is extended straight to the ventral extremity and tends to be differentiated into a moderately inflated main part and a somewhat flattened posterior part. Although there are some differences, I. kamuy has affinities with I. atlanticus, I. nodai and probably I. rutherfordi of middle to late Cenomanian age. These species may be affiliated with a group originated from I. virgatus and, thus, belong to Inoceramus in a broad sense. On the other hand, I. kamuy is not likely to be ancestral to the I. (I.) hobetsensis and I. (I.) Iamarcki groups of middle Turonian age. In view of some similarity and variability in shell shape and ornament, we suggest that I. kamuy may be an indirect source from which, in the late Turonian, I. teshioensis, I. costellatus, and I. perplexus (with rather uniformly convex valves) and I. longealatus (with posterodorsally flattening) may have been derived. However, the linking forms should be sought in the middle part of the Turonian. Several species of latest Turonian to Coniacian age, such as I. frechi, I. glatziae and I. multiformis, are seemingly similar to I. kamuy in some of their particular characters, but their phylogenetic relationships with I. kamuy have yet to be worked out.
