Abstract.-The buoyancy, stability, and orientation of a shelled cephalopod in water are the predictable
 products of shell geometry, body chamber length, and such physical parameters as shell, tissue, and water
densities.  Given such physical characteristics as shell geometry, shell, tissue, and water densities, and shell
thickness, the hydrostatic characteristics of planispiral shelled cephalopods, induding orientation, centers
 of mass and buoyancy, stability, and neutrale buoyant body chamber length, can be calculated and
simulated using microcomputer-based techniques.  Individual variables such as geometry, body chamber
 length,and shell thickness are linked in a calculable manner to orientation, neutral buoyancy, and stability.
 Living Nautilus provides a means of testing the model and for making hydrostatic comparisons between
 ammonoids and nautiloids.  The close agreement between calculated versus observed body chamber lengths
in five species of Mississippian ammonoids shows that neutral buoyancy, and (with one exception) Nau-
 lus-like orientations, were at least feasible for these species.