Andrew, I think you are suffering from a case of taxonomia! The oldest name is the right one?? That works in taxonomy, but not in common english; otherwise we would all be speaking Olde English :-) I think "mollusk" is still in commonest usage among the general population, while "mollusc" is usually used by those of a more scientific bent, presumably because it is derived from the phylum name Mollusca, which does NOT have any alternate spellings. Paul M.