The identity of<i>Gastrochaena cuneiformis</i>Spengler, 1783, and the evolution of<i>Gastrochaena, Rocellaria</i>, and<i>Lamychaena</i>(Mollusca, Bivalvia, Gastrochaenoidea)

<jats:p>The specimens of<jats:italic>Gastrochaena cuneiformis</jats:italic>Spengler, 1783, with Spengler-written labels at the Zoologisk Museum, Copenhagen, did not come from Spengler's type locality in the Nicobar Islands, and may instead be syntypes of Chemnitz's (1788) West Indies<jats:italic>“Pholas hians”.</jats:italic>the identity of<jats:italic>Gastrochaena cuneiformis</jats:italic>as a senior synonym of<jats:italic>Gastrochaena gigantea</jats:italic>(Deshayes, 1830) is established on the basis of Spengler's original descriptions and illustrations, and by examination of specimens from the type locality. A neotype for<jats:italic>G. cuneiformis</jats:italic>is designated and illustrated, and its genus is revised to exclude<jats:italic>Rocellaria</jats:italic>Blainville, 1829, and<jats:italic>Lamychaena</jats:italic>Freneix<jats:italic>in</jats:italic>Freneix and Roman, 1979.<jats:italic>Gastrochaena</jats:italic>Spengler, 1783 is the most plesiomorphic of these three genera, as shown by its simple boring, short siphons, and diffuse, poorly differentiated anterior pedal muscles.<jats:italic>Rocellaria</jats:italic>evolved from a close common ancestor with<jats:italic>Gastrochaena</jats:italic>, and is characterized by a ventral shift and fusion of the posteroventral pallial sinus with the posteroventral pallial band, low, irregular posterior commarginal lamellae, and well defined anterior pedal retractor muscles generally supported by myophores.<jats:italic>Lamychaena</jats:italic>evolved from<jats:italic>Rocellaria</jats:italic>during the Oligocene, extending its ctenidia far posterior into the siphonal part of the boring, and, in some species, uniting its anterior pedal retractor and protractor muscles as they approach the byssus apparatus.</jats:p>