Thanks Gary and Helmut, I realized that there are many genera that differ by only one letter - Partulida - Partulina Mucronalia - Mucronaria Siphonaria - Siphonalia Columbella - Columella Mytella - Mysella Tectula - Tectura Marisa - Marita Suterella - Suterilla Tortulosa - Torulosa But in all these cases, the difference occurs somewhere other than the final ending denoting the gender of the noun. I wondered specifically whether it makes any difference when the change occurs in that ending, since then, in effect, the two names are really the same word, simply altered to give a different gender (as in Nassarius - Nassaria). However, since the ICZN article Gary quoted didn't mention any exception for this situation, I assume it must be included in that ruling. Paul M.