The oolitic limestone of the North York Moors was laid down in a warm tropical sea about 160 million years ago in the Jurassic Period. In close-up, oolite resembles tiny round white fish eggs, hence the name.
The spiral shells are, as you might suspect, molluscs, but the thick mussel-shaped shells are brachiopods, also known as lamp shells because of their resemblance to Roman oil lamps.
I spotted them in the walls of Beech Farm Cottages, Wrelton.