The Cretaceous deposits of the Western Interior were laid down in a fluctuating sea that at times extended from Mexico to the Arctic. High land on the west supplied much detritus, low land on the east supplied little. Two biotic realms contributed to life in this sea—northern temperate, perhaps cold temperate, and southern warm, perhaps tropical. At times a large endemic element was present. The faunas, as preserved, are dominantly molluscan. The observable ecologic data are the character of the fauna and the kinds of sediment. These are discussed for 17 epochs, chosen from a standard time scale. In late middle Albian time the sea invaded the region, extending from the south to Montana. Intermittent depression of the embayment produced in the south an alternation of barren black muds and shell-bearing calcareous muds and in the north black muds with fauna restricted progressively northward. These deposits were covered by sand, implying filling of the embayment or withdrawal of the sea. The sands contain air-borne volcanic material prenuncial of the next epoch. In late Albian time the sea advanced southward to northern Colorado, and an enormous amount of volcanic ash, possibly from central Idaho, was deposited. Life of the epoch is poorly known, though disintegrated remains of fishes are found everywhere, and at a few places there are invertebrates related chiefly to northern stocks. In early Cenomanian time southern waters connected with those from the north, and dark rather barren muds were laid down. In perhaps middle Cenomanian time a few forms of southern origin appeared, but they were succeeded by faunas of northern derivation. In late Cenomanian time a fauna from the south again dominated and is associated with a change to deposition of calcareous muds that implies reduction in the amount of land detritus. Volcanic activity to the west continued, though with decreased intensity. Early Turonian time brought widespread deposition of calcareous muds and a cosmopolitan fauna. Middle and late Turonian times appear to have been epochs of variability in conditions; volcanic activity was negligible, and part of the fauna was cosmopolitan, part endemic. The latest Turonian brought great restriction of the seaway. Coniacian time was the epoch of greatest expansion of the sea and the second of widespread deposition of calcareous muds. In the east the lime muds were purer, and shelled animals rare. Toward the west more land detritus was brought in, and faunas were varied. Volcanic activity revived, though it was not intense. Similar conditions continued into early Santonian time. In late Santonian and early Campanian times the sea was greatly restricted east and west, and toward the north the deposits were sandy. The faunas appear to have come from the south, and connection with boreal waters is not apparent. Local volcanic activity began in west-central Montana, and some volcanic ash was carried widely. In the middle Campanian the sea expanded greatly, and the deposits were mostly rather barren black muds. In the early part of the late Campanian filling of the sea-way or a shallowing in the west carried sands well out toward the east, and a varied fauna, mostly immigrant from the south, appeared in the east. In the latest Campanian the seas again expanded westward, and a varied fauna existed, marked particularly by aberrant ammonites. Mild volcanic activity to the west continued through the Campanian. In Maestrichtian time the sea began its final withdrawal, and the western shore line moved eastward until eventually only a small area of sand deposition was left. The fauna included both southern and endemic forms. Finally nonmarine conditions prevailed over the entire Interior Region.