This effect should be strongest when the excurrent region of the shell faces into the current. Orientations where the anterior-posterior axis is perpendicular to the current direction should result in ...

Michael Savarese, Functional Significance of Regular Archaeocyathan Central Cavity Diameter: A Biomechanical and Paleoecological Test, Paleobiology, Vol. 21, No. 3 (Summer, 1995), pp. 356-378 ...