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The Omma Formation, distributed in Toyama-Ishikawa area, Hokuriku province, is well known because the formation yields abundant and particular marine molluscan fossils which constitute the so-called "Omma-Manganzi" fauna of Northern Japan. The writer undertook detailed stratigraphic work on the Omma Formation in order to define precise stratigraphic positions of fossil beds from the unit in the Toyama-Ishikawa area. Abundant fossils were collected from more than 200 localities. As the first step in the study, the writer determined and described the thickness and lithology of the shell beds, feature of shell beds, state of shell preservation, number of species, specific composition and combination of species. After compilation of the above data, an attempt was made to interpret the paleoecology and sedimentary environment of the Omma Formation based fundamentally upon the association of the dominant species and lithology of each fossiliferous bed. Analysis of the various fossiliferous beds of the Omma Formation indicated that five distinct assemblages could be recognzied. These assemblages and their associated lithologies are as follows : Acila-Anadara-Nemocardium-Limopsis-Clinocardium-Venericardia-Macoma assemblage of the silty sand facies; Anadara-Clinocardium-Felaniella-Glycymeris-Thracia-Peronidia-Rexithaerus assemblage of the fine grained sand facies; Felamiella-Glycymeris-Peronidia-Clinocardium-Mercenaria-Pseudamiantis assemblage of the medium grained sand facies; Glycymeris-Felaniella-Peronidia-Anadara-Venericardia-Clinocardium assemblage of the coarse grained sand facies; and Callithaca-Peronidia-Glycymeris-Felaniella-Mercenaria-Dosinia assemblage of the granule to pebble facies. These assemblages are considered to correspond to one sedimentary cycle of marine transgression. Brief remarks on 80 representative species of the Omma Formation are made, among which one genus, four species and one subspecies are described as new to science.

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