Dear Rob;-
    Well, sure. But the modifiers are not always nouns. Rare-formed
is adjectival to Epitonium. Like Fly specked would be to a cone. And
I'm not sure that Queen Conch does not use "Queen" as an adjective
describing the conch. The other guy who answered quite well used the
terms "common" and "simple". "Common" I can accept. But what may
appear to be "simple" to some, may be a bit more complicated to
others.
     Hecky-darn, I'm not going to argue with the majesty of ICZN,
whether we know who they are or not. I had just never heard the term
"specific epithet" before. And I'm really not all that new to the
study of shells. I have heard it called the "specific" name, or the
"trivial" name. Maybe it makes a lot more sense in Latin.
       Art