The microstructure of the shell of bivalve molluscs was examined by lightand electron-microscopy. The purpose of this paper is to classify and describe this structure, and to discuss the affinity and evolutionary development among different kinds of shells. Shell structure consists of two elements, namely, the morphological type which is divisible into 11 different kinds, and “growth line”. The distribution of each morphological type in pclecypods is summarized. Various combinations of morphological types are divided into four major groups. The origin of these types and groups is discussed.