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 previously described species of Adontorhina, the new species are more elongate and less inflated.