The inner shell surface is the biomineralization site in shell formation and an inner-shell film covers it.This surface is composed of two regions:an outer calcitic region and an inner aragonitic region.In this study,some amalgamated calcite crystals were found in the calcitic region and some aragonitic “imprints” were found in the central part of the aragonitic region.The “imprints” are probably the trace of mantle cells that adhered to the inner shell surface when the shell was produced.Furthermore,to build a novel in vitro biomineralization system,the inner-shell film was detached from the shell and introduced to the calcitic crystallization solution.Crystallization experiments showed that nacre proteins could induce aragonite crystals in the novel system but inhibited calcite growth in the absence of the inner-shell film.These data suggested that the inner-shell film may induce aragonite growth in vivo by combining nacre proteins. The inner shell surface is the biomineralization site in shell formation and an inner-shell film covers it. This surface is composed of two regions: an outer calcitic region and an inner aragonitic region. In this study, some amalgamated calcite crystals were found in the calcitic region and some aragonitic “imprints ” were found in the central part of the aragonitic region. “ imprints ” are probably the trace of mantle cells that adhered to the inner shell surface when the shell was produced.Furthermore, to build a novel in vitro biomineralization system, the inner-shell film was detached from the shell and introduced to the calcitic crystallization solution. Crystallization experiments showed that nacre proteins could induce aragonite crystals in the novel system but inhibited calcite growth in the absence of the inner -shell film.These data suggests that the inner-shell film may induce aragonite growth in vivo by combining nacre proteins.