Abstract
The large, sand-burrowing Indo-West Pacific mussel “Modiola “ vagina Lamarck has been placed in the mytilid genera Modiolus and Botula by different authors. Dissection of freshly collected live material has shown that it possesses long, extensible siphons with complex internal structure. In this respect it resembles the lithophagine genus Botula rather than Modiolus. It also resembles certain other siphonate sand-burrowing mussels that form byssal cocoons and which are variously classified at present in the Modiolinae or Crenellinae. However, the species has a set of shell and anatomical characters that distinguish it from both Modiolus and Botula, and from other siphonate mytilid genera, warranting the erection of a new genus to contain it. The affinities of the new genus, Arenifodiens, are discussed with the conclusion that assignment of it to one of the present mytilid subfamilies is not advisable until a full review of the family has been completed.