Morphological change in growth process is a phenomenon restricted to organism and it is important to clarify the mechanisms of morphological change and factors affecting it. The purpose of this study is to investigate the factors controlling morphological change based on theoretical considerations of growth process and biometrical analysis of shell form. In general, since the growth rate of an organism is considered to be proportional to its size, the weight of organisms is expressed as an exponential function of its volume. Furthermore, introducing a concept of density, it is expected that the change of density with growth is either proportional or inversely proportional to the volume of organisms. Biometrical analyses of shell form were carried out in order to test the result of theoretical con sideration. The specimens use were fossil samples of Anadara kogachiensis from the Pliocene of Motobu Peninsula in Okinawa Prefecture. The relative density and weight of shell were calculated from the measured value of shell and the variation of density throughout growth process was computed. The result shows that the density variation has three phases; it decreases gradually in early stage of growth, then increases in middle stage. This result indicates that the morphology of shell is closely related to a function of organism which tries to keep the density constant and also suggests that gravity is an important external factor affecting shell morphology.