Thin, light-weight valves of Mulinia lateralis, a lagoonal clam, are found evenly and naturally interbedded in clay at 8 to 33 ft. above mean sea level in a clay dune of the mainland shore of Laguna Madre, the coastal lagoon of southwestern Texas. The dune is on the lee shore of a small embayment where the shells seem to have been carried from the parent lagoon by waves and currents and strewn on a mud flat normally barren of subaqueous invertebrates. The shells were then redistributed by wind over the dune. Previously, the only fossils reported from clay dunes have been Foraminifera and food animals and shells of aboriginal campsites. The environment of the occurrence is briefly described, with a review of the origin and development of clay dunes. An irregularly distributed layer of organic and inorganic flotsam deposited on the clay dunes by storm waves is also described, and it is concluded that the M. lateralis shells were not similarly deposited.