What are these?

[Keyslivin]

I am hoping you can help me with this. My husband and I were scuba diving off the ocean side of the Florida Keys at about 30'. I often like to dig in the sand while diving to see what kind of treasures I can uncover. Yesterday, I came across these interesting treasures and have no clue what they are. The are domed and on the flat bottom there is a swirl, like a shell. The are solid like a rock. Have you any clue? They were about 1' under the sand.