The zone of Exogyra cancellata forms the lower part of the Exogyra costata zone in the Upper Cretaceous and ranges in thickness from 4 feet in New Jersey to 200 feet or more in central Texas; in terms of the European section it is upper Campanian or lower Maestrichtian in age. This zone has been traced, with interruptions, through various formations, from the Navesink Highlands of New Jersey to Cardenas in the state of San Luis Potosi, Mexico. In Texas the zone forms the lower part of the Navarro formation, overlying the Taylor marl, and overlain by the Nacatoch sand member of the Navarro. From New Jersey to central Texas the zone is characterized by Exogyra cancellata and Anomia tellinoides, both of which are restricted in their stratigraphic range to the zone. A dozen o more of the other molluscan species that occur in the zone, but are not restricted to it, have the same extended geographic range. This assemblage indicates a fairly uniform set of environmental conditions from the northern end of the Atlantic Coastal Plain to the Gulf region. The occurrence of rudistids in the Exogyra cancellata zone in Texas indicates a tropical climate there, and points to a climate not colder than subtropical in eastern North America as far north as New Jersey. The presence of the zone at the surface in structural uplifts on the flanks of the Palestine, Keechi, Rayburn, and Prothro salt domes, indicates an extended down-dip spread of the zone, which at the Rayburn dome reaches a known maximum of 115 miles.