Other macroinvertebrates in the Niobrara fauna include baculitids, Clioscaphites choteau- ensis Cobban, certain rare ammonites, cuttlefish, belemnits, and free-swimming crinoids, all of which were probably inhabitants of more freely circulating, better oxygenated surface waters. Crinoids, cephalopods, and planktonic foraminifers all suggest fully marine conditions during Smoky Hill deposition. Deposition of Smoky Hill sediments was relatively rapid, at least in some parts of the section. Tall cone-shaped ecovariants of Ostrea congesta, found in the same beds with the encrusting form, apparently were attached to small shell fragments. The growth form is assumed to reflect the rapid upward growth that was necessary to prevent burial. Oysters observed on baculitids in two horizons are all small, suggesting that burial occurred not long after attachment. Inoceramus shells encrusted by large numbers of oyster spat are pre- sumed to have been dead at or soon after the time of epizoal attachment and were buried before the oysters grew very large. In contrast, Inoceramus shells heavily encrusted by thick- shelled adult oysters were alive during oyster growth, and remained unburied for lengthy periods. On such Inoceramus valves, second-generation oysters are commonly attached to the shells of the earlier oyster generation. The end of a fascinating chapter in Cretaceous history came with the major influx of terrigenous detritus into the western Kansas area at the beginning of Pierre deposition. This sediment heralded the second major Late Cretaceous regression of the Western Interior Sea in Kansas. Pierre Shale* The Pierre Shale is present in only a few counties in western Kansas where its total thickness is about 1,400 feet. The formation is largely dark-gray to medium-gray shale that contains gray- to brown-weathering limestone concretions, rusty- to dusky-red-weathering ironstone concretions, and thin layers of white or gray bentonite. Elias (1931) described in detail the outcrops in Wallace County where the lower one-half or less of the Pierre Shale is present. This part of the Pierre was divided by Elias into the following members, from oldest to youngest: Sharon Springs (155 ft.), Weskan (170 ft.), Lake Creek (200 ft.), and Salt Grass (60 ft.). *Section on Pierre Shale prepared by W. A. Cobban Figure 5.- Common macroinvertebrate fossils in Niobrara Chalk and Pierre Shale. (1) Ino- ceramus grandis (Conrad), Smoky Hill Chalk Member of Niobrara, X2/3; (2) Ino- ceramus deformis Meek, Fort Hays Limestone Member of Niobrara, X2/3; (3) Ino- ceramus platinus Logan, Smoky Hill Chalk Member of Niobrara, actual size is 35 x 34 inches; (4) (5) Baculites asperiformis Meek, Sharon Springs Shale Member of Pierre Shale, both X2/3. 22