Paul et al.,
 
As you suspected, a noun that is used as a species name is invariant. This
is called a noun in apposition, and it is not actually used as an
adjective. Nouns (or pronouns) in apposition are equivalent, as in "I,
Claudius", where I = Claudius, or "Farmer John". So in Conus terebra, the
meaning is clear: "Conus = terebra".
 
In "tree frog", "tree" modifies "frog"; the tree is not the frog, so these
nouns are not in apposition. Latin never uses nouns to modify other nouns
as in this English example, so if you see a noun used as a species, either
the author intended it as an appositive noun or he didn't know his Latin.
However, there are some cases where a Latin noun and adjective have the
same ending, and in such a case you have to go back to the original
description and hope that the author discussed its etymology. If the
original author said nothing about it, then it's up to the first reviser to
make an arbitrary decision, and that word is final.
 
Cheers,
Andrew
 
Andrew K. Rindsberg
Geological Survey of Alabama