Shell microstructures of brackish-water bivalvian molluscs were examined by the use of light and scanning electron microscopes. The species examined were Corbicula japonica and Geloina coexans, Corbiculidae.
　 Their layer structures are discriminated into a periostracum and a calcareous shell layer. The latter is subdivided into two layers, namely outer and inner layers, and moreover, has myostracum.
　 In the shell of Corbicula japonica the calcareous shell layer is composed of the outer finely crossed lamellar and inner complex crossed lamellar layers. The thicker inner layer near the umbonal side has fine canal structure. In the shell of Geloina coaxans the periostracum is also subdivided into four sublayers under light microscopy and intruded into outer calcareous shell layer at some places. The calcareous shell layer consists of the outer crossed lamellar and inner complex crossed lamellar layers. Their shell layers are made of calcium carbonate mineral, aragonite.
　 Growih structure is well marked in calcareous shell layers , composed of wider dark and narrower light bands arrenged alternatively. The texture of each morphological shell structure is not cutted by any their bands. The growih bands of Geloina is more clear than those of Corbicula . The cause of their differences is due to environmental effects.