FIGURE 1. Diagrammatic transverse sections comparing the posterior portion of the ligament and nymphae of Acharax and Solemya. A, the upward-arching (parivincular) external ligament and nymphae, i.e., the plesiomorphic condition as seen in Acharax. (Drawing based on Carter, 1990, fig. 17D; Bailey, 2011, pl. 4, figs. 4-6; and Taviani et al., 2011, fig. 5). B, external nymphae of Acharax with the ligament removed. When the ligament is not preserved, the nymphae are visibly separated by an internymphal gap. C, hypothetical intermediate condition showing beginning stages of ligament depression and inward rotation of nymphae (arrows). D, submarginal ligament and nymphae - the apomorphic condition seen in Solemya. The depressed ligament is secondarily overgrown by a shelly outer prismatic layer. (After Carter, 1990, fig. 17A.) E, the submarginal nymphae of Solemya with the ligament removed. Notice the outer secondary layer that covers the former internymphal gap. Abbreviations: ne - external nymph, ns - submarginal nymph gr - ligament insertion groove, ig - internymphal gap, lgf - fibrous inner layer of ligament, lgl - lamellar outer layer of ligament, pe - periostracum, opl - outer (secondary) prismatic shell layer.