SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION Emphasis upon publication as a means of "diffusing knowledge" was expressed by the first Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series: Smithsonian Contributions to Anthropo/ogy Smithsonian Contributions to Astrophysics Smithsonian Contributions to Botany Smithsonian Contributions to the Earth Sciences Smithsonian Contributions to Paleobiology Smithsonian Contributions to Zoology Smithsonian Studies in Air and Space Smithsonian Studies in History and Technology In these series, the Institution publishes small papers and full-scale monographs that report the research and collections of its various museums and bureaux or of professional colleagues in the world cf science and scholarship. The publications are distributed by mailing lists to libraries, universities, and similar institutions throughout the world. Papers or monographs submitted for series publication are received by the Smithsonian Institution Press, subject to its own review for format and style, only through departments of the various Smithsonian museums or bureaux, where the manuscripts are given substantive review. Press requirements for manuscript and art preparation are outlined on the inside back cover.

The Triassic fossil record points to the monophyly of the Pectinoidea (scallops), all members of which have a triangular resilium with a nonmineralized medial core that functions below the hinge line. The elastic properties of this resilium in extant taxa predict that the initial adaptation of the Pectinoidea was to swimming. This is indeed corroborated by the shell form of Pernopecten, the earliest known pectinoidean genus, which ranged from late Devonian to earliest Triassic. The new family Entolioididae, a largely Triassic group, provides the missing link between the Pernopectinidae and the families Propeamussiidae, Entoliidae, and Pectinidae, all of which originated by the Middle Triassic and survive to the present day. A new Triassic genus Filamussium shows that the Propeamussiidae originated from the Entolioididae, not directly from the Pernopectinidae as previously supposed. Evidence from morphology, the fossil record, and molecular genetics indicates that the family Spondylidae originated in the Middle Jurassic from an ancestor within the Pectinidae, possibly the genus Spondylopecten, which was already present in the Late Triassic. Journal compilation © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society, 2006, 148, 313–342. No claim to original US government works

SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION Emphasis upon publication as a means of "diffusing knowledge" was expressed by the first Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series: Smithsonian Contributions to Anthropo/ogy Smithsonian Contributions to Astrophysics Smithsonian Contributions to Botany Smithsonian Contributions to the Earth Sciences Smithsonian Contributions to Paleobiology Smithsonian Contributions to Zoology Smithsonian Studies in Air and Space Smithsonian Studies in History and Technology In these series, the Institution publishes small papers and full-scale monographs that report the research and collections of its various museums and bureaux or of professional colleagues in the world cf science and scholarship. The publications are distributed by mailing lists to libraries, universities, and similar institutions throughout the world. Papers or monographs submitted for series publication are received by the Smithsonian Institution Press, subject to its own review for format and style, only through departments of the various Smithsonian museums or bureaux, where the manuscripts are given substantive review. Press requirements for manuscript and art preparation are outlined on the inside back cover.

Two morphological paradigms have long been used in comparative anatomical studies of bivalves: (1) the primary ligament is three-layered, with the layers corresponding to three shell layers; and (2) the primary mantle edge is composed of three folds with clearly defined functions. The results of studies of larval development indicate, on the contrary, that the primary ligament is completely organic. Calcified, fibrous ligamental material develops from lamellar material near areas of contact between ligament and shell, and development of the fibrous portions then proceeds toward the midline, finally achieving in many lineages a continuous fibrous bridge between valves. Furthermore, these results suggest that the mantle edge in the Bivalvia is primarily twofold and that the only clearly homologous structure between major groups is the periostracal groove itself. These morphological concepts, with other new and previously published data on shell ultrastructure, ligaments, mantle edges, ctenidia, palps, lips, stomachs, muscles, and photoreceptors, lead to a new picture of the evolution of primitive and derived character states in groups previously included in a subclass Pteriomorphia. Furthermore, a cladistic analysis of these data allows predictions of the morphology of ancestors which can be tested by reference to the preserved morphology and sequence of fossils. A new phylogenetic classification separates these groups into three superorders within the subclass Autobranchia: Isofilibranchia (mytiloids), Prionodonta (arcoids), and Pteriomorphia. The most complex radiation has been in the Pteriomorphia. Three orders originated in the early Palaeozoic: Pterioida, Limoida, and Ostreoida. The Pterioida and Ostreoida developed monomyarian, pleurothetic states independently, and each order developed its own mode of shell secretion. Further differentiation in the order Ostreoida occurred in the mid-Palaeozoic, producing two suborders, the Ostreina and Pectinina, both of which had already developed foliated calcitic ultrastructure from simple prismatic structure. By the early Mesozoic, the Ostreina had given rise to three extant superfamilies - the Ostreacea (true oysters), Dimyacea, and Plicatulacea - through atrophy of the foot, the assumption of a pleurothetic state on either the left or right side, and early obligate cementation. The Pectinina, through retention of the foot and the assumption of a pleurothetic mode of life, had evolved before the late Palaeozoic to the Anomiacea and Pectinacea. Within the superfamily Pectinacea, four extant families have origins ranging from early Carboniferous to Cretaceous in age: Propeamussiidae, Pectinidae, Syncyclonemidae, and Spondylidae. The new family Syncyclonemidae, which contains a genus long assumed to have become extinct at the end of the Cretaceous, is here recognized in the Recent and late Pleistocene on opposite sides of the Earth. With regard to extinct groups, many genera previously assigned to the Pteriacean family Malleidae belong in the Ostreacea on the basis of shared derived character states. Incorporation of these taxa as well as the Dimyacea in the Ostreina suggests that oysters have a dimyarian, possibly non-pleurothetic, origin and cannot have evolved from forms like the Pseudomonotids, which retained their foot and became pleurothetic. The new name Buchiacea is introduced for a set of extinct taxa within the suborder Pectinina including the Buchiidae, Monotidae, Oxytomidae, and Pseudomonotidae of previous authors. Derivation of this group is from the common ancestry of the Anomiacea and Pectinacea. The extinct Palaeozoic Aviculopectinidae, Pterinopectinidae, Deltopectinidae, and Leiopectinidae are grouped in a superfamily Aviculopectinacea, which also appears to have branched from the early ancestry of the Pectinina.