Samuel Pepys ate oysters from 'a barrel', but in reality the barrel was about 12 inches high and soaked in salt water before the small London oysters were packed into it for transit.  They would keep like that for a couple of days and were opened for gentlemen to eat as they stood at the bar, small knives and picks being provided for the purpose.  Today was a nice day so I walked on the foreshore and found some nice examples of oyster shells.  The ones at the bottom are the small London oysters of the 17thC, the ones Pepys would have eaten.  By the middle of the 18thC, the London stocks were dying out due to pollution and Thames traffic and other sources had to be found.  Essex began to supply the London trade, with wagons and carts making the trek daily.  The two in the middle are Essex oysters, served from carts pushed along the streets, made hot with pepper and vinegar.  The top one is the large 'Scuttlemouth', a big shell for a small, sweet oyster very similar to the London ones, but sourced from the South Coast and brought daily by trains after 1850.  

Oysters are rich in protein and zinc, and so were a healthy addition to a diet that may sometimes have lacked high quality meat and dairy products.  The foreshore is covered with thousands of these shells, testifying to London's massive appetite for this raw morsel.  It was a food eaten by every level of society: street vendors talk of supplying trays of shucked oysters for dinner parties at grand houses, at the same time as serving the parson his supper of 6 oysters to supplement his cheese on toast.