Archaeolithothamnium is a genus of the coralline algae. The genus is of interest because of its primitive structure, its long geologic history and numerous fossil representatives. Appearing in the late Jurassic, numerous and quite varied species developed during the Cretaceous and probably it reached its zenith in the late Eocene. It continued abundantly until the late Miocene. Since then it has declined steadily until today it is represented by about 15 species. Over a hundred fossil species have been described. The structural features of the genus are described, and its ecology, geologic history, geographical distribution, and possible phylogenetic position are discussed. The essential data for determining the species are tabulated. Tables give this data for the species of each period (or epoch of the Tertiary), and for the Recent.