Help me identify this shell from Sanibel Island

[dave r]

Yes, after storms is possibly the best way to collect beached shells, a lot of deeper water species get brought up which you would not normally see (your junonia for instance) I often go to the beach here in England after a good storm to see what has ben cast up and have had some good results.
There is a system for grading shells; G=Gem, F=Fine, G=Good, C=Commercial ( or some would say Crap!), most shell books or websites will have this, but it is open to interpretation. A 'Gem' quality cowry for example should be absolutely flawless but there are species of shells that are never found in perfect condition, so you have a set of shells that are never 'Gem'. Some people quote 'Gem for species' which means it is about as good as it gets. Some Murex shells have a few tiny breaks on the fronds which are almost invisible, you have to look carefully or even use a lens to see them. I personally would say this is Gem but purists would not. Some also put a plus or minus after the grade to mean that it is 'not quite' or 'slightly better than' but this can lead to nit-picking.
As to small,perfect specimens or big flawed ones, I think it is all a matter of personal choice. At the end of the day it is your collection and whatever pleases you is the best.
Dave