The Senonian series of the Languedocian furrow contains, as in all of southern France, reef formations with rudistes. One species, Hippurites (Orbignya) socialis Douvillé alone is responsible for practically all of the building. The sequencial analysis of such formations permits the reconstitution of the morphology of the reef, lenticular and very flat. Some pioneer individuals colonize a mud, then they unite to make a building, but this is highly perforated and “spongy”. The products of mechanical demolition and biological corrosion (algae, sponges) accumulate at the periphery, and may also be colonized. The building dies due to its becoming covered with bioclastic, quartzitic or mixed sand. In the regional context, the recifal building is situated on the edge of an actively subsident furrow, at the boundary between a littoral domain (shelly sands) and a “deep” domain with rhythmic sedimentation. There is a very unstable equilibrium between sedimentation and subsidence, and the recifal phenomena establishes itself under conditions rarely encountered.