Dilemma japonicum new species (Bivalvia: Anomalodesmata: Poromyidae): A new record of the genus from the Northwest Pacific

The fourth species of the bivalve genus Dilemma Leal, 2008, is described from disarticulated valves collected off central Japan. The discovery of these specimens represents a significant range extension for the germs to the Northwest Pacific. The new species is distinguished from the other three known species by its surface sculpture, shape of escutchcon, and hinge. The prodissoconch, indicative of lecithotrophic development, and shell microstructure, with outer homogeneous and inner nacreous layers, are illustrated for the first time for a species of the genus.