This is a fossil I found when I was a kid (Boy that was a fast 35 years). It is about 5x6.5 inches and has hundreds of shells on it. My friend dropped it and I found that the shells are all through it.
If you can make it down to Florida, there are a few places still open to the public to collect fossil shells. They find a lot of them while mining for phosphate, such as Cargill.
Sometimes you're better off attending a field trip thru a shell show or club, where you get a guide to take you to the good spots.
I went to four different spots, and some were so loaded with shells that are extinct now, you could fill a 5 gallon bucket up in two minutes. One of these days I'll take some out and post some pics.