If you go beach combing surely one of the most obvious and easily identified shells you may come across is the razor clam (Ensis ensis). Looking a bit like a 'cut throat' razor it is not hard to see how it got its common name! There are two similar, but less common, species.
Able to bury itself deep into the sand or silt for protection it is more likely to be found on sandy beaches than rock shorelines and it is quite common right around the British Isles but in Dorset Studland beach is the best place to find it. When disturbed at low tide, even by the vibration of approaching feet, it is a very effective burrower and so is rarely seen alive unless one goes digging for it, you are most likely to encounter the empty shell on the sand of the beach at low tide. As it burrows it ejects a spout of water and leaves a distinctive keyhole shape in the sand which is a it of a give away to its presence! If it is seeking to hide from danger leaving a trademark indicator is not really such a good idea.
There are various recipes for cooking razor clams on the Internet so they are obviousness edible so if you have a liking for shell fish go looking for those keyhole shapes in the sand at low tide. You will have to dig deep, they can burrow to to half a metre deep.