The depth to which an intact coiled cephalopod shell will sink with its plane of symmetry vertical is related to shell geometry. Equations that relate shell geometry to physical constants of pressure, sea-water density, and gravity yield the maximum depth of stable verticality of a shell. Water depths calculated from the geometries of vertical shells of Cymatoceras hilli (Shattuck), Drakeoceras drakei Young, and Mortoniceras wintoni (Adkins) from the Fort Worth Formation (Upper Cretaceous, Albian) of north-central Texas are used to reconstruct a portion of the paleobathymetry of the East Texas Embayment during late Fort Worth time. The maximum water depth to which shells with these geometries would have sunk with the plane of symmetry vertical range from 1.6 m for the least stable shell geometry (D. drakei) to 2.6 m for the most stable shell geometry (C. hilli). Water depths were deeper than 2.6 m at localities where no vertical shells were observed and shells with the plane of symmetry horizontal showed no evidence of being reworked from the vertical orientation.