The atlas-axis complexes of four of the major groups of the mammal-like reptiles are discussed. These represent the four principal types of occipital condyle found in the Synapsida: hemispherical in the pelycosaurs, kidney-shaped in the gorgonopsids, trefoil-shaped in the dicynodonts and double in the cynodonts. In each case the possible movements of the skull are analysed in detail. It is shown that the central theme of the wide variation of this structure found within the synapsid reptiles is the manner in which the problem of achieving rotation of the head about a longitudinal axis was solved.
An attempt is made to elucidate the functional evolution of the atlas-axis complex within these reptiles and the significance of the differences between the reptilian and the mammalian complexes is noted.