Ligament and lithodesma in the Pandoracea and the Poromyacea with a discussion on evolutionary history in the Anomalodesmata (Mollusca: Bivalvia)

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Abstract
Certain families in the subclass Anomalodesmata possess highly characteristic ligaments exhibited in simplest form in the Lyonsiidae (Pandoracea) and in the Verticordiidae (Poromyacea). A ventral ly displaced primary ligament consisting of inner (fibrous) and outer (lamellar) layers lies beneath a dorsal secondary ligament of fused periostracum. The latter, which occupies the pivotal axis, is responsible for alignment of the edentulous valves, the former for production of the opening thrust. A calcareous lithodesma which extends down the centre of the inner layer (and so also formed by the mantle isthmus) ensures the continued effectiveness of the widened ligament. Conditions in the laterally compressed Pandoridae and in Myadora (Myochamidae) resemble those in the Lyonsiidae with the addition of secondary teeth. Cementation by the right valve causes bilateral asymmetry in Myochama but to a far greater extent in the Cleidothaeridae where the added effect of a tangential component in shell growth causes separation of primary and secondary ligaments and a twisting of the lithodesma. In the Verticordiidae and Cuspidari‐idae (Poromyacea) ligamental structure is the same although modified in the latter. In the Poromyidae the primary ligament remains external and there is no lithodesma while fused periostracum forms anterior and posterior ligamental extensions.Despite these resemblences in ligamental structure between Poromyacea (pre‐septi‐branchs and septibranchs) and Pandoracea, palaeontological evidence, as well as the modified ctenidia in the former, supports the continued separation of these superfamilies. On the other hand the Thraciidae, Periplomatidae and Laternulidae, hitherto included in the Pandoracea (but originating earlier, in the Mesozoic instead of the Cenozoic) should be removed from that superfamily. In them the primary ligament is not ventrally displaced and an anterior lithodesma is solely concerned with alignment of the edentulous valves to which it is closely attached.

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