AMERICAN cookbooks contain a wealth of clam recipes but few for mussels. French cookbooks concentrate on mussels, and rarely mention clams at all.

Exactly why is a mystery, because both types of mollusks are available in America and France. Luckily for cooks in both countries, clams and mussels are similar in many ways; most recipes suited to one are also good for the other. For example, both are delicious steamed and added to creamy pasta sauces, or baked with garlic butter.

Mussels do have a slight advantage in their beautiful color. Their shiny blue-black shells and bright orange or yellow flesh make them perfect for garnishing otherwise pale fish dishes. Generally, though, if you are used to preparing clams, there is no reason not to use mussels.

Both mussels and clams are easy to cook and relatively inexpensive. A treat to enjoy even in the simplest of restaurants in France is a dish piled high with moules a la marinie re, plump fresh steamed mussels with shallots and white wine. It is as common there as clam chowder is here.

Actually, neither clams nor mussels should really be cooked, but rather opened through the action of heat. As soon as they open, they should be removed from the heat, because further cooking would toughen them and make them rubbery. Very fresh mussels and small clams are so tender that they are sometimes served raw, like oysters.

Some mussels recipes might seem perplexing because they require cooking the mussels without water, but then call for using the cooking liquid in the sauce.. The reason is that mussels hold sea water inside them. When heat forces them open, this liquid comes out. Clams need a little more coaxing to open than mussels do and are therefore cooked with a small amount of liquid. They open when the liquid boils.

The cooking liquid of both clams and mussels is prized in the kitchen because of the good flavor it adds to seafood sauces and soups. In fact, clam juice is even sold here in bottles so it can be used as a substitute for fish stock and to flavor sauces when clams are not available or not needed for the dish.

Saltiness is characteristic of clams and mussels, and especially of their cooking liquid. For this reason, a little of this liquid goes a long way and should be added gradually when flavoring a sauce. Salt should never be added to clams or mussels as they cook.

For both types of mollusks, smaller generally is better. Large clams tend to be tough, so try to choose littlenecks or ipswich. In France, small mussels from Normandy and Brittany are considered best.

The strong flavor of mussels and clams can be balanced by combining them with other ingredients with plenty of character, such as garlic and mustard, or with acid ingredients such as dry white wine and tomatoes. The freshness of herbs also complements their taste, especially relatively robust herbs such as thyme, oregano and basil. Adding rich ingredients such as cream, butter or mayonnaise, or using the mollusks in a sauce for pasta or rice, are effective techniques that tame their vigorous flavor.

In addition to switching clams and mussels in recipes, the cook can pair both in the same dish, to give an interesting combination of colors, tastes and textures. Next time you prepare clam chowder, use fresh clams and try adding a few mussels; together, they'll give a wonderful natural aroma of the sea. PREPARING CLAMS AND MUSSELS FOR COOKING

Both clams and mussels should be cooked live. If any of the shells are already open, tap them gently against the sink or other surface; the shell will close again if the animal is still alive. If it remains open, discard it. Clean clams and mussels thoroughly before they are cooked. Put them in a colander and rinse well several times under cold running water; do not leave to soak in the water. Scrub clams thoroughly with a stiff brush. Use a knife to scrape mussels clean of foreign particles and pieces of the "beard" that joins them together. After mussels are cleaned, their shells should look bright and shiny.

If, after a batch of clams or mussels is cooked, some don't open, they should be discarded; they were not alive when they were put in the pot. BAKED CLAMS OR MUSSELS WITH GARLIC BUTTER (4 servings)

In French this appetizer is referred to as clams or mussels "a l'escargot" because the same garlic butter is used for baking snails. Garlic butter: 1/2 cup (1 stick) unsalted butter, softened 4 large cloves garlic, very finely chopped 4 tablespoons chopped parsley Pinch of pepper For the clams: 2 pounds littleneck clams or other small clams or mussels 1/2 cup water (for cooking clams)

Make the garlic butter: Thoroughly beat the ingredients with a wooden spoon or mash them together with a fork until blended.

Prepare the clams or mussels for cooking (above). Discard any open mussels that do not close when tapped. Put mollusks in a large saucepan. If using clams, add the water. Cover and cook over high heat, shaking the pan often, about 5 minutes or until the mollusks open. Discard any that do not open. Remove from the cooking liquid. When cool enough to handle, remove the top shell of each. Transfer the clams or mussels in their bottom shells to a baking dish; set them on a bed of coarse (kosher) salt or on crumpled foil if necessary, to prevent them from rolling.

Divide the garlic butter among the clams or mussels. Bake in a 425-degree oven about 7 minutes until the butter melts and the mollusks are very hot. Serve immediately in the shells. PASTA WITH CREAMY CLAM OR MUSSEL SAUCE (2 servings)

The strong-flavored clam or mussel cooking liquid, softened with cream and herbs, makes a simple but wonderful sauce for pasta. 1 pound small clams or mussels 1/4 cup dry white wine 1/2 cup whipping cream 1 1/2 teaspoons chopped fresh basil or 1/2 teaspoon dried basil 4 ounces fettucine Salt and pepper 1 tablespoon chopped parsley

Prepare the clams or mussels for cooking (see above). Discard any open mussels that do not close when tapped. Put the mollusks in a medium saucepan and add the wine. Cover and cook over medium-high heat, shaking the pan often, about 5 minutes or until the mollusks open. Discard any that do not open. Remove from the cooking liquid, but reserve the liquid. Leave to cool; shell the clams or mussels. Cut the clams in small pieces, but leave the mussels whole.

If the cooking liquid is sandy, leave it undisturbed for 10 minutes; then carefully pour it into another medium saucepan, leaving the sand behind. Bring the cooking liquid to a boil. Stir in the cream. If using dried basil, crumble it and add it with the cream. Simmer over medium heat, stirring often, about 7 minutes or until the sauce is thick enough to coat a spoon.

Meanwhile, cook the fettucine in a large pan of boiling salted water for about 5 minutes or until al dente. Drain thoroughly.

Return the clams or mussels to the sauce and reheat very briefly. Add the fresh basil and parsley and a pinch of pepper and taste for seasoning. Pour over the pasta and toss. Serve immediately. CLAM OR MUSSEL AND CELERY SALAD (3 to 4 servings)

The mustard-flavored dressing resembles a light mayonnaise and complements the taste of both mollusks and celery. 1 1/2 pounds small clams or mussels 1/4 cup water (for cooking clams) 4 celery stalks 1 red bell pepper or large tomato Mustard dressing: 1 egg 1 tablespoon dijon mustard 1 tablespoon white wine vinegar 1/2 cup vegetable oil Pinch of pepper

Prepare the clams or mussels for cooking (see above). Discard any open mussels that do not close when tapped. Put the mollusks in a large saucepan. If using clams, add the water. Cover and cook over high heat, shaking the pan often, about 5 minutes or until the mollusks open. Discard any that do not open. Remove from the cooking liquid. When cool enough to handle, shell the mollusks.

Using a vegetable peeler, peel the celery to remove the strings. Cut the celery in thin slices. If using a bell pepper, cut it in thin strips; if using a tomato, cut it in thin wedges.

To make the dressing, blend the egg, mustard and vinegar in a food processor or blender. With the machine running, gradually pour in the oil in a thin, steady stream. Continue processing until the dressing is smooth. Season with pepper to taste.

To serve, mix the celery with half the dressing. If using clams, place on top of the celery mixture and coat each with a little dressing. If using mussels, mix them with the celery and add enough dressing to moisten the mixture. Decorate with the pepper strips or tomato wedges. Serve at room temperature. CLAM AND MUSSEL STEW WITH TOMATOES AND WHITE WINE (2 main course or 4 appetizer servings)

In this colorful dish, the clams and mussels are served in their shells. Instead of a mixture, clams or mussels alone can be used. Serve with toasted sourdough or french bread. 2 tablespoons olive oil 1 small or 1/2 large onion, chopped 1 clove garlic, chopped 1 1/2 pounds ripe tomatoes, peeled, seeded and chopped 1/2 cup dry white wine 1/4 teaspoon thyme 1 bay leaf Freshly ground pepper to taste 1 pound mussels 1 pound small clams 1/4 cup water 1 tablespoon chopped parsley

Heat the olive oil in a heavy frying pan. Add the onion and cook over low heat, stirring often, until soft but not brown. Stir in the garlic and cook for 1/2 minute. Add the tomatoes and stir for 2 minutes over medium-high heat. Pour in the wine and add the thyme, bay leaf and pepper. Cook, stirring often, for 12 to 15 minutes or until thick. Discard the bay leaf.

Meanwhile, prepare the clams and mussels for cooking (see above). Discard any open mussels that do not close when tapped. Put the mussels in a large saucepan. Cover and cook over high heat, shaking the pan often, about 5 minutes or until the mussels open. Remove the mussels with a slotted spoon; discard any that do not open. Cover the mussels to keep them warm. Carefully pour the mussel liquid into a bowl, without adding any of the sand remaining at the bottom of the saucepan. Discard the sandy liquid. If the cooking liquid in the bowl is still sandy, leave the liquid undisturbed for 10 minutes; then carefully pour into another bowl, leaving the sand behind.

Put the clams in the saucepan and add the water. Cover and cook over medium-high heat, shaking the pan often, about 5 minutes or until they open. Remove the clams with a slotted spoon; discard any that do not open. Reserve the cooking liquid.

Gradually add half the mussel liquid and half the clam liquid to the tomato mixture. Taste; if a saltier taste is desired, add more mussel and clam liquid. Heat the mixture again until slightly thickened but not dry. Add the mussels and clams in their shells, cover and heat briefly. Serve in bowls; sprinkle each serving with chopped parsley.