The Flagstaff formation of central Utah is divided into three units; the lowest is Paleocene, the middle is Paleocene or Eocene, and the upper is Eocene. The lowest unit contains 28 species of fresh-water and land Mollusca; the middle unit contains 2 species of land snails; the upper unit contains 13 species of fresh-water and land Mollusca. The total molluscan fauna consists of 37 species—5 fresh-water pelecypods, 24 fresh-water gastropods, and 8 land gastropods. Four new species are described: Elliptio mormonum, Lampsilis spiekeri, Hydrobia ephraimensis, and Carinulorbis utahensis. The quantitative and volumetric abundance of each species is given. The paleoecology of the lake is examined from the following standpoints: size of the lake, depth of water, chemical composition of the water, fluctuations of the shore line, surrounding land surface, source of sediments, nature of sediments, wave action, evaporation, vegetation, composition and variation of the molluscan fauna. The history of the lake is described from its origin in late Paleocene time to its extinction in Eocene time. In the first phase, the lake was small, with abundant vegetation and a large molluscan fauna. In its second phase, it deepened and expanded considerably; the gypsum content of its waters increased, and the molluscan fauna was wiped out; only land snails and fragments of molluscan shells were preserved. In the third phase, the lake was still large but shallow; a new molluscan fauna invaded the lake and persisted until its extinction. The Flagstaff lake was partly wiped out by encroachment of alluvial-plain sediments and partly merged with the Green River lake which spread into its area from the north.