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Triplofusus giganteus
large snail found in ocean, seven inches in length
ocean
The way you are holding the shell is hard to tell if it is a welk shaped shell (with a "flatter" top) or a conch (with a very pointy spiral). but I'm pretty sure this is the Florida state shell the Horse Conch. see a similar picture here:
http://www.adventuremike.com/tag/horse-c...
scroll down a bit.
And here is a real cool one of a horse conch actually eating a whelk!
http://dixiehwy.blogspot.com/2009/06/pal...
You might want to look into the Knobbed Whelk. I'm not entirely sure since other species shells can look very similar.