A model is presented to determine the critical shear stress for the incipient motion of bivalve shells on a horizontal sand bed, under a unidirectional flow of water. Hydrodynamic forces on a solitary bivalve shell, resting over a sand bed, are analyzed for the condition of incipient motion including the effect of turbulent fluctuations. Hydrodynamic forces such as drag and lift are taken into consideration. Three types of bivalve shells, namely Coquina Clam, Cross-barred Chione and Ponderous Ark, were tested experimentally for the condition of incipient motion. The shape parameter of bivalve shells is defined appropriately. Experiments were conducted in a flume with a horizontal bed, and the critical shear stresses were computed using Vanoni's side-wall correction. The experimental data are used to calibrate the model making lift coefficient a free parameter. The results obtained using the present model agree satisfactorily with the experimental data.