Fig. 1 in The origin of ammonoid locomotion

Fig. 1. Forces operating on ammonoids during swimming, parameters, and terminology. A. Forces operating on ammonoids during swimming (modified from Jacobs and Chamberlain 1996). The thrust force produced by the jet which is expelled by the hyponome acts on the centre of gravity. This causes an oblique downward momentum which is opposed by the restorative moment (resulting from buoyancy and gravity) and the drag. At relatively high velocities, this might result in a fairly stable horizontal movement in some derived ammonoids. B. Angles of the body chamber length and of the orientation of the aperture. C. Terminology.