The description of the mollusks in the Tertiary formations of California and Baja California is continued from Chapter B. The families Plicatulidae, Spondylidae, Anomiidae, Limidae, Gryphaeidae, and Ostreidae, which represent 23 genera and 68 species, are covered in this chapter; other chapters will follow in zoologically systematic order. All the Tertiary species that were described from or are known to occur in the Californias are included, along with their original descriptions and their distribution by formation. The holotype is illustrated where feasible, and tables show the geologic and geographic occurrence of the species in each genus. Of the 23 genera studied, the Gryphaeidae and Ostreidae were most in need of taxonomic revision, and only 1 of the 39 species originally assigned to Ostrea is retained in that genus. The marked response of oysters to their particular habitats causes more variation in shell character within individual species than in any other group of bivalves. The size, shape, and sculpture are all affected by substrate, by crowding, and even by the intensity of available light. The abundance and gigantic size of Crassostrea and Striostrea during Miocene time in California parallels that of the pectinids during the same time period, just prior to the well documented cooling trend that reached its nadir during the Pleistocene. This population and gigantism explosion may be related to shallow, restricted seaways fed by rivers and streams rich in nutrients, in addition to the known relationship to the temperature of the seas.