Glycymeridid bivalves from Japan and adjacent areas. Part II,• Indo-Western Pacific species of Melaxinaea Iredale, 1930 (Pliocene to Recent)

Abstract

The glycymeridid bivalve genus Melaxinaea (Pliocene to Recent) was originally based on a Recent Queensland, Australia, species M. labyrintha Iredale, 1900, which may be a junior synonym of M. vitrea (Lamarck, 1819). The genus Melaxinaea is characterized by its strongly compressed shell, two straight rows of hinge teeth meeting at an angle, and divided and intercalated nodulose ribs and inner ventral crenulations. It should be placed in Tucetona group rather than in Glycymeris s.s. group in the Glycymerididae, because shells of Melaxinaea and Tucetona are ornamented with well-defined radial ribs and are without a velvety periostracum and fine striations on the shell surface in which radial rows of well-developed periostracum are inserted. In addition, their ligamental areas are always incised by distinct ligamental grooves. The genus Melaxinaea is however more specialized than Tucetona, because the ribs and inner ventral crenulations become divided and intercalated in ontogeny. The type material of the following species of Melaxinaea, Tucetona, and Glycymeris are illustrated: M. vitrea, T. pectiniformis, T. hanzawai, T. subpectiniformis, and G. capricornea.