This paper deals with a paleoecological analysis of the molluscan fauna of the Hekikai formaton based on fossil mollusks found in the coarse sandstones of an outcrop at Takahama-cho. The molluscan fauna contains one hundred and twenty-eight species, as listed in Table 1. In general, the fauna is a past Japonic-Kuroshio fauna and consists of one main assemblage, Standella-Rapana (inhabitants of a sandy or silty bottom) and two subordinate ones, Nipponarca-Bedevina (adhering or crawling forms on a gravelly or rocky bottom) and Anomia-Ostrea (sessile forms on rocks or gravels). Faunal resemblances with the Quaternary and living faunas in the other districts have ben investigated. The results are presented in Table 2, Fig. 3 and Fig. 4 and the conclusions are made as follows ; 1) The fauna of the lower Atsumi formation (muddy matrices) reveals a high degree of taxonomic resemblance to the fauna of the Hekikai formrtion, in spite of the differences in age and matrices of the both formations. 2) The constituent of molluscan fauna of the Hekikai formation strongly resembles that of the Atsuta, Noma, Kozakai, Sahama, Kusanagi and Uemachi formations of the same age. 3) Faunas obtained from the formations of distant localities, i.e. the Hiradoko (sandy matrices), the Takinogawa and Kuchinotsu formations are low in taxonomic resemblance although the sediments of the same age. 4) In comparison with the molluscan fauna of Ariake Bay, the fauna of the Hekikai formation bears a stronger resemblance to the fauna of the inner part than that of the outer part of the bay. The following sedimentary environment factors are assemable at the environs of Takahama in age of the Riss-Wurmian Interglacial. 1) Place : middle or inner part of an inland sea or small bay along the coast of an inland sea. 2) Bottom character : coarse to medium-grained sands with gravels. 3) Salinity : normal salinity of sea water ; estuary or brackish bay existed in the neighbourhood. 4) Depth : sublittoral, perhaps about 15 m. 5) Water and current : coastal sea water, influence of a warm current (past Kuroshio) similar to or slightly stronger than that of the present Kuroshio.
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