In this paper the author roughly reviews the content of former studies on the inner shell structure of bivalvian molluscs. Nextly, the shell structures of them are discussed with results of his studies. It has been examined by many microscopists since the end of 18th century. Recently, not only the micro-shell structure has been fairly cleared under an electron-microscope, but also the process of shell formation and mechanism of calcification have been discussing on the base of experiments using isotope-chemical and histo-chemical methods. Bivalvian shells generally consist of aragonite or calcite crystal layers, bearing organic matters called conchiolin. Calcareous layers are divided into some layers of different structure, generally composed of outer and inner layers. According to the microscopic observation under lower magnifing power, the inner shell structure can be divided into ten basic types, namely periostracum, prismatic, fibrous prismatic, nacreous, foliated, crossed lamellar, complex, homogeneous, pellucid and canal struetures. Until now, the author examined some micro-structures of them, especially on crossed lamellar, complex and canal ones. The crossed lamellar structure of O.B. BφGGILD has been clarified by the writer that it has first order lamellae consisting of the aggregation of second order lamellae. It is observable that these second order lamellae are composed of the aggregation of rectangular aragonite crystals. The breadth of them is 0.5 microns or so, but its length is unknown. He supposed that organic matter occupy around these crystals like bandage and scatter in them. The complex structure is also composed of similar rectangular crystals of aragonite. The structural feature is shown in Fig. 9 as to Glycymeris. The canal structure, which had been appointed by former investigators in some pelecypods, is observable also by the writer over much more species, such as Arcacea, Sj5ondylus, Chama and Cardita. The diameters of these canals are ranging from 0.5 to 10 microns. Though the writer observes that cells of mantle intrude into these canals, whose function of them has not yet been clarified. Electron-microscopic-photographs in the writer's plates show the pattern of crystal from on the inner surface. The form of crystal on the inner surface of nacreous and foliated structures is very clear, but the form of them is different one another. The distribution of basic types in the Filibranchia group is listed as follows. 1. Arcacea ; crossed lamellar and complex st., 2. Mytilidae ; fibrous prismatic and nacreous st., 3. Pteriidae and Pinnidae ; prismatic and nacreous st., 4. Anomiidae ; prismatic and foliated st., 5. Pectinidae; foliated st., 6. Ostriidae; prismatic and foliated st., 7. Spondylidae; foliated, crossed lamallar and complex st., 8. Limidae; foliated and complex st. Periostracum and pellucid structure are omitted.