About 300 jointed bivalves of Glycymeris yessoensis were collected from a lenticle of an almost unispecific shell bed in the Onma Formation, central Japan. Most of the valves were partly open and completely infilled with the surrounding sandy silt; 5 were sealed and contained only a small amount of aragonite-cemented siltstone. The void in the chamber of the sealed valves is occupied with acicular needles of aragonite overgrown on the inner surface of the valves and on the surface of parasitic boreholes within the shells. The fibrous aragonite on the inner surface of the shells is in optical continuity with the aragonite crystals at both inner and outer structural layers. The fibrous aragonite indicates strong depletion of O¹⁸ and slight depletion of C^{13 UP> compared with the shell. A cold to temperate open sea comparable with that off the western coast of Hokkaido at present is zoogeographically indicated for the Onma molluscan fauna. These isotopic depletions differ from that of aragonite cement generally found in grapestone clusters, reef rocks, and beachrocks, all of which are typically tropical. The textural evidence suggests that precipitation of the aragonite postdated the partial infilling by sediments, but took place when the sediments were plastic; hence a freshwater origin of the aragonite is excluded. A plausible interpretation is aragonitic growth from a solution trapped and warmed within the chamber during an early stage of fossilization. Aragonite cementation may occur in a localized space such as a shell chamber in nontro ical seawater.}

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