Cream 6 tablespoons of the butter with the shallots, garlic, parsley, celery, salt and pepper. Remove the mushroom stems and reserve for another purpose. Heat the remaining butter in a skillet. Add the mushroom caps and turn to coat on all sides. Arrange in the depressions of a snail pan, in scallop shells or in a shallow baking-serving dish. Place a scant teaspoon of the herbed butter in each mushroom cap, add a snail and cover it with a little more butter. Before serving, bake in a moderate oven (375 degrees) about 15 minutes. NOTE: Any leftover herbed butter may be used for baked or broiled fish.