Two new species of Adontorhina Berry, 1947 (Bivalvia: Thyasiridae) from the Porcupine Bank, off the west coast of Ireland

A minute bivalve, Adontorhina keegani, new species (Thyasiridae) is described from the Porcupine Bank, west of Ireland. The new species occurs between 300 and 789 m on the continental slope. The shell is elongate and compressed, with a flattened posterior and complete lack of radial sulcus. The shell has a hinge margin bearing irregular granules and noticeably low umbones, located in the posterior. Hydroids were found growing on the margins of the shell, indicating a shallow burrowing habit. Adontorhina similis, new species, was previously recognized as a European variety of Mendicula pygmaea Verrill & Bush, 1898, but is here shown to be a distinct species. The hinge bears irregular granules, which precludes classification as a Mendicula species. The shell is elongate, moderately inflated with prominent umbones and a pointed posterior margin. The posterior flank of the shell is flattened and does not bear hydroids. In comparison with pre-viously described species of Adontorhina, the new species are more elongate and less inflated.