Dear James;-
   You may certainly describe a species IF:
    1. You have enough examples of that species to know that it is
constant and not just a variation of some other species.
    2. You have compared your "new" species with all the species that
are or may be close, and can distinguish the characteristics of the new
species.
    3. Have submitted the "new" species to experts on that family who
will agree that you do, indeed, have a new species.
    There aint no short cuts. And there is never any hurry in taking the
step you plan.
     If you like, I can probably come up with 4, 5, and 6. Be aware that
many newly described species of mollusks simply were NOT compared to the
species they were closest to because the describer did not have
sufficient resources to find those species. You must also be absolutely
sure that your "new" species is not just a variation of a species that
has already been described as "variable".
   With all that firmly in mind---go ahead.
     Art