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Data from the monographic literature indicate a general size reduction amongst Bathonian marine bivalves in southern England: mean size is reduced in all subclasses and major mode-of-life categories relative to the immediately preceding Bajocian stage (27 per cent. reduction) and the later Oxfordian stage (41 per cent. reduction). Smaller size cannot be explained in terms of sedimentary facies, nor as a ‘pure’ evolutionary phenomenon (involving no environmental change); it therefore probably reflects some quality of the ambient water. A comparable reduction in average size (19 per cent. relative to Bajocian, 46 per cent. relative to Oxfordian) is evident amongst oyster and scallop lineages studied throughout Europe and a sedimentary-facies (and pure evolutionary) control can be ruled out at least for an area extending from southern England to the east side of the Paris Basin. A lowering of salinity to the mid-twenties per mil is the likeliest cause of size reduction in this area. Isolation of the region from the oceans, combined with high fresh water runoff, was probably the determining factor in such widespread lowering of salinity. High runoff may have been caused by increased rainfall. Tests are proposed for the ubiquity of the pattern identified, and the suggested proximate and ultimate causes. It is noted that recognition of widespread reduced salinity has implications for the salinity tolerances of certain ‘stenohaline-marine’ taxa and the salinity ranges ascribed to certain nearshore faunal associations. In addition, recognition that environmentally determined size reduction may occur over a broad area has implications for the general issue of interpreting phyletic size change.