On the Rocks

The tide was very low this afternoon, revealing plenty of rocky shoreline. In an area that sees few human visitors and is some distance from the high tide line, I found several of these large shells. They were all securely fastened to the rocks and firmly closed.This was the first time I have come across these, so I had to include one for my blip today.  The bright sunshine and high contrast made it difficult to get a good shot though. As to what they are, I don't really know.  My first thought was that they were clams, though possibly they are oysters, or maybe neither?