The west coast of North America has had marine environments particularly conducive to the accumulation and preservation of the most extensive deposits of diatomaceous sediments in the world. Inasmuch as this area can furnish dependable geological occurrences and, in turn, accurate stratigraphic ranges of diatom species from the Cretaceous to Recent, it can eventually become the standard of the world for diatom biostratigraphy. The following twelve floras have been described, they are: Late Cretaceous, 155 species, 136 restricted; late Eocene, 42 species, 19 restricted, largely planktonic species; early Oligocene, 51 species, 21 restricted, decreasing number of open-sea forms; middle Miocene, 82 species, 54 restricted, largely benthonic forms; late Miocene, 47 species, 7 r stricted, mainly neritic to sub-littoral; late Miocene, 69 species, 24 restricted, mainly planktonic; late Miocene, 96 species, 52 restricted, mainly benthonic forms; late Miocene, 280 species, 145 restricted, mainly vagile benthonic and sessile littoral forms; early and middle Pliocene, 150 species, 70 restricted, planktonic forms in the lower and middle portion with bottom dwellers increasing in number in the upper portion; early Pliocene, 59 species, 18 restricted, mainly planktonic species with a minor number of brackish-water forms; late Pliocene, 101 species, 19 restricted, mainly planktonic species with a minor number of fresh-water forms; early Pleistocene, 136 species, brackish and fresh-water forms. The diatoms also serve as a satisfactory check on the paleoecologic significance of diatomaceous deposits, not only because most diatoms are restricted to certain environments with regard to temperature, pH, and salinity, but also because they are restricted as to mode of life, i.e., planktonic, neritic, vagile, and sessile benthonic, littoral, marine, brackish, and fresh water. End_of_Article - Last_Page 1092------------