This sauce can be made with clovisses, the stripy clams abundant in the Bassin de Thau which many people will know as vongole, or with the larger palourdes. Another excellent clam is the amande or dog cockle. Lastly there is the praire or venus clam, ridge-shelled and much enjoyed in Languedoc. Clovisses have beautifully marked deep purple and fawn shells, are about 2 cm across, and feel satisfyingly heavy in your hand. Locally they are made into this rich fricassee to serve with rice or pasta.
To keep clams alive, which they must be until you cook them, take them out of their plastic bag, wrap them in a wet cloth, put them in a bowl, cover it lightly with a disc of cling film laid on the cloth to keep them from drying out, and keep them in the refrigerator. Cook them on the day of purchase if at all possible.