Figure 11 in Palaeoheterodont diversity (Mollusca: Trigonioida + Unionoida): what we know and what we wish we knew about freshwater mussel evolution

Figure 11. Diagram of patterns of posterior mantle fusion types in the Palaeoheterodonta. In the diagrams, the darker, outer layer represents the outer fold of the mantle, and the inner layer is the inner fold. The middle sensory fold is greatly reduced in the Unionoida. See text for discussion. e, excurrent aperture; i, incurrent aperture; sa, supra-anal aperture.