Fossil Hunters at Pigeon Lake

While out at our good friends' cottage on Pigeon Lake, Ontario, my five-year old nephew and I went for a walk. These days he's always picking up rocks. Since there is a lot of shale on the beach there, I suggested we look for fossils.
A few minutes later, and bingo! My nephew has his first fossil. Our hosts were gracious enough to let us keep it.
It appears to be a shell, and it looks similar to a cockle, perhaps like a small specimen of Acrosterigma? (Thank you, Dorling Kindersley handbooks!) Trying to identify it together at home was half the fun.