Place clams in a large bowl. Discard broken shells or clams that won't close. Fill bowl with cold water to cover. Sprinkle cornmeal over clams, and stir. Let sit 3 to 6 hours to allow clams to purge sand from their shells.
Remove clams from water, and rinse thoroughly. Set aside.
Place a steamer insert into a large (16-quart) stockpot, and fill halfway with water. Add Old Bay, garlic, and onions, and bring to a boil over high heat. Reduce heat to simmer, and add potatoes and next 4 ingredients. (Make sure ingredients are completely submerged in water.)
Cover and cook 10 to 12 minutes or until potatoes are tender. Add clams. Cover and cook 4 to 5 minutes or until clams open. Discard any that do not open.
Spoon into serving bowls, including extra cooking liquid, if desired. Serve with French baguette, melted butter, and additional Old Bay.