The mode of life of the Japanese Miocene bizarre pectinid Nanaochlamys notoensis (Yokoyama, 1929) is postulated on the basis of functional morphologic and biometric examinations of a large sample from the Middle Miocene Moniwa Formation in northern Japan. This species exhibits a remarkable ontogenetic change of shell morphology; i.e., the weight of valves becomes negatively allometric to the cube of shell size, indicating relative thinning of the shell with growth. Following this allometric change, the umbonal angle increases, the auricles become symmetrical, and the ctenolium disappears. Comparison of shell morphology with Recent pectinids strongly suggests that the present species changed its life style from byssally attached to free-living during ontogeny. This pectinid probably could swim in the later growth stage, but its swimming ability appears to have been poor in view of the high convexity of the left valve. Secondary flaring of the shell and splitting of the first-order plicae are apomorphic features in N. notoensis, suggesting that this species is not a direct ancestor of the extant North Pacific pectinid Swiftopecten swiftii, with seemingly similar overall shell morphology.