The labyrinthodont Amphibia flourished from Upper Devonian times to the Rhaetic. Amongst them is a series of related forms-covering, perhaps, the longest time span to be found in fossil materials-whose earliest member is Actinodon from the French Coal Measures, and latest the cyclotosaurs from the Upper Trias of several countries. The material is good, most of it is South African, but important specimens come from many other parts of the world. The skull shows little change with time in the structure of the dorsal surface, but the palate and braincase change progressively, and the process is described. Some of the skulls have associated lower jaws, which are figured, and may help in the understanding of isolated lower jaws. Postcranial material is too rare to be taken into consideration. The series is important because its long range in time and worldwide geographical distribution make it valuable in the determination of the horizons of the rocks in which its members are found. Two new genera and two new species have been established.