I suppose, thanks to Molly Malone wandering through the street of Dublin selling them, that cockles and mussels are are best known shell fish. They are also the most common. A walk along Studland beach will reveal the familiar empty shells that have washed up from the sandy bay; they are the only ones with growth rings running along the shell rather than in concentric circles.
The common cockle (Cerastoderma edule) was once very common in sandy bays and estuary mud flats but are less so now due to over predation by human beings who, strangely in my view, seem to like to eat them I tried them once and thought they were made of rubber soaked in salt and vinegar. That said I have memories of childhood holidays with my uncle, who lived in Scotland, where we would go cockle digging, shell them and hen use them as fishing bait - we seemed to have fsh for dinner every night.
Cockles can bury themselves in the sand at low tide but only down about six inches. When the tide is in and they are on the top of the sand to feed they can, apparently, jump to avoid predators; not a lot of people know that!