Visceral sac and shell coiled in one plane; foot divided transversely into two parts, posterior part bearing an operculum, anterior part forming a fin provided with a sucker.
Lung-fish (Ceratodus) was the most primitive (or Archipterygium), placed as the primordial type a fin which palaeontology has proved to be one of the latest types if not the last.
First, when used as a fin for progression; Second, when used as a mace in battle; Third, in sweeping; Fourth, in lobtailing; Fifth, in peaking flukes.