In the various localities of the Cretaceous System in Japan, many trigonian species, about 50 or more, have been described by many authors, from a variety of conditions of environments such as brackish (?), subtidal, littoral and neritic. This paper deals with the stratigraphical occurrences and presumable inhabited environments of the trigonians. One of the conclusions of this study, the stratigraphy of each trigonian, is shown in Text-fig. 1. This is very useful for the stratigraphy of the Cretaceous System in Japan especially in the faunas which are very poor in the occurrence of ammonite and inocerami. Another one, the inhabited environment of each trigonian, is shown in Text-figs. 4 and 6. It seems that the number and feature of the costae on the disk, outline of the valve and feature of the area in the trigonians, are very closely related to the conditions of inhabited environments of the trigonians.
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