Florida Horse Conk

They have deadly spears

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-conch/ 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/palm-pavilion-and-clearwater-marine.html

You might want to look into the Knobbed Whelk. I'm not entirely sure since other species shells can look very similar.