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.