The Tuxtla or Ocozocuautla formation has been attributed to various ages from Upper Jurassic to Eocene, and there has been controversy whether it lies below or above the thick rudist limestone which, under various names, has been referred to different ranges between Comanchean and Maestrichtian inclusive. A paleontological succession covering the range upper Turonian to Campanian has been published, but without reference to the lithological succession. In this paper it is shown that the Ocozocuautla series is above the rudist (Sierra Madre) limestone, its lithological and paleontological successions are described, and correlated with each other and with the standard succession. The Sierra Madre limestone is shown to embrace the range from Lower Cretaceous to Turonian, the Ocozocuautla series being Campanian and Maestrichtian. No evidence was found of the occurrence of Coniacian or Santonian rocks, which are thought to be absent. The discrepancies between the conclusions reached in this paper and those of earlier authors are discussed, and their reasons investigated.