The Terasoma Formation is a part of the Shimanto Group in Wakayama Prefecture (Fig. 1). It consists of shale or shaly flysch, with pebbly mudstone or massive sandstone, in the lower member (200-350m), sandy flysch in the middle member (350m), and laminated shale or massive mudstone in the upper member (250m) (Fig. 2). The strata are intensely folded. Five localities near Terasoma (indicated with solid circles in Fig. 1), two (locs. 22 and 29) in the lower member and three (locs. 04, 05 and 01) in the upper member, have yielded a considerable number of inoceramid specimens, which are classified into six taxa as listed in Tables 1 and 2. They are described in Japanese and illustrated (Pls. 1-4). The faunule is peculiar to the Cretaceous of Japan. Inoceramus (Platyceramus) cycloides vanuxemiformis, I. (P.) rhomboides rhomboides and I. (P.) rhomboides heinei indicate that the main part of the upper member is of Santonian (probably early half of the Santonian) age, while I. (Magadiceramus) sp. nov. (?), allied to I. (M.) subquadratus, suggests that the same member could possibly comprise the uppermost Coniacian in its lower portion. Mytilodes sp. cf. M. sublabiatus or cf. M. mytilodiformis from loc. 22 and an ammonite Yubariceras aff. kanei (Jones) (Pl. 4) from loc. 29 (somewhat below 22) suggest the Turonian age of the lower member, although its upper portion could be Coniacian. Mytilodes sp. nov. (?) from the upper member is an interesting example of the genus in the Santonian, but too poorly preserved to define a species. An ammonite from loc. 04 is Hauericeras (?) sp. aff. H. angustum. Some discussion is given on the palaeoenvironment of the Terasoma Formation.
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