THE MERRIMENT surrounding Mardi Gras already has begun for this year's observance planned March 4. This post card of a Mardi Gras float dates from 1950. It notes in part, "Papier-mache floats depicting episodes from mythical, historical or legendary themes and manned by members of the various carnival organizations make up a series of street pageants. To this, add the thousands of enthusiastic revelers and spectators lining the route of march and you have something of the 'Spirit of Mardi Gras.'" From the Bessie Gatchel Collection.