Well, I'm going to simply offer this as a suggestion, because it's quite possible your shell got that way naturally. But, I want to point out that sometimes shells were used to impress designs in clay before it was fired. I have a specific example in mind, but don't have a photo, of a dentate stamp a friend found. See the quahog shell over on the right side of the illustration? Well, a friend found one in which there was a series of square notches placed in the curved leading edge of the shell. If placed on wet clay, you would rock the shell and it would thereby impress a series of square punctuate designs in the clay. You can see other types of dentate stamps on this page. Again, not saying those are not natural elements in your shell fragment, but only pointing out sometimes the edges of shells were altered to create a dentate stamp...
Charl, if you held this in your hand you might lean more toward this being altered by man. It was found at a known site and I didn't see shells of a similar age/thickness around. There is a good amount of pottery and some points. I've seen a lot of shells, broken and whole. This could be natural but based on the circumstances I think I will keep it. Thank you for your input.
I am sorry for the poor pictures but my phone is old. These are some of the shell tools which have a very sharp angled edge. The last pic is interesting as it is the center of some conch type with a chisel like edge ground on it. The center pic has a small plummet as well. I liked Charles post as well. Who knows what these Native Americans utilized at times? I bet we would be surprised. These were found in the Titusville Fl. area in the 80's. I have more but the pics didn't turn out well at all.
Thanks for the pics Newnan. I've seen similar at some museums in SWFL. Yes, there is no telling the number of uses and variety of tools. If I saw your shell tools just laying in some dirt/sand I wouldn't give them a second glance ...but if they were on Mound Key or a preserve on Marco...well, that's a different matter. I drove down to Marco post-Irma after reading of all the shell tools exposed in the roots of upended trees at a preserve. Those shells are carbon-dated to 700-900 yrs ago - not hugely old but very cool all the same.