The status of the Plicatulidae and the Dimyidae in relation to the superfamily Pectinacea (Mollusca: Bivalvia)

In the families Pectinidae and Propeamussidae (scallops) the massive rounded inner ligament layer which provides the opening thrust is bounded by long stretches of anterior and posterior outer ligament layer which, in the absence of teeth, maintain alignment of the valves. In the Spondylidae these outer layers migrate centrally to unite on either side of the inner layer, the primary ligament becoming transversely instead of longitudinally disposed. A secondary ligament of fused periostracum with no tensile strength unites the still long hinge line but secondary ball and socket teeth assume responsibility for alignment of the valves. In the Plicatulidae and the Dimyidae there is the same transverse disposition of the primary ligament but the mantle margins overarch this with production of a continuous longitudinal “external” secondary ligament above the “internal” primary ligament. Although resemblances with the Spondylidae are greater than with the Pectinidae and Propeamussidae, differences from all three are great enough to justify separation of the other two families into a new superfamily Plicatulacea. The monomyarian condition in the Plicatulidae could well have arisen from conditions similar to those in the Dimyidae, that is following cementation when dimyarian and not following byssal attachment as it does in the three families of the now proposed restricted Pectinacea.