ID conundrum - how old is too old?

[wateryworld]

So, I'm going through the book again after doing other things for a bit, and the very first page I go to has this problem.  It's showing the melongenas, and calling the perverse whelk (busycon perversum) and the lighting whelk (busycon contrarium), as two different species.  From what I've tried looking up, they are actually one single species, those two names supposedly actually being synonyms.    Can anyone help clarify?  This sort of thing is starting to drive me seriously nuts.