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