The Atlantidae is a group of holoplanktonic gastropods, which all demonstrate a strong adaptation to pelagic life, in the form of a lenticular, laterally flattened, aragonitic shell, the surface of which is further enlarged by the presence of a wide, and very thin and fragile, double-walled keel. The apex is on the right side of the shell, the umbilicus on the left. For the living animal the enlargement of the shell’s surface serves ‘to increase stabilization during swimming and sinking’. This phenomenon results in the occurrence of superficially very similar adult shells in separate species, whereas the larval shells may be utterly different.