I recently took a trip to Sanibel island, and Key Marco.  I brought back some lightning whelk shells.  My first tool project was to make a shell hammer.  Based on the blade, I call it an ax.  I am still learning about these shell tools, so calling it a hammer seems kind of goofy for what it really seems to do.  
Shell:  Busycon perversum
Handle:  Osage orange
Wrap: rawhide
Once it is good and dry I will find a tree to chop and see how it does.