Presence of both prosogyrous and opisthogyrous forms among primitive pelecypods leads to confusion in orientation of extinct species. Because the protobranch foot protrudes from the anterior portion of the shell, the muscles controlling foot movement bear direct relation to shell orientation. Accessory muscle scars in Missississippian species of extinct genera Ctenodonta and Nuculopsis are analogous to those in many living genera, but differ from Ordovician Ctenodonta. It is suggested that pedal muscle scars have been reduced in number throughout the history of taxodont protobranchs. --V.M.J