It's an Ark Clam (Arcidae). Can't tell which one, but that's all you normally see, the rest is embedded in the rock. It's pretty common to find the back sections of the clam (the part you can see in the pic) lying on the sand or washed up on the beach. Either fish or some other animals apparently bite off the exposed end with some frequency. They will usually withdraw until they are almost flush with the rock, if touched.
Cheers,
Don