Biostratigraphy of Neogene faunas from the Humboldt basin of northern California emphasizes six foraminifera aggregates encountered in the thick clastic sections. Other groups, notably mollusks, are included.

Detailed analysis of the chronologic and chorologic relations of the component elements (lineages, faunas, and stratal units) provide a framework for comparison and correlation of clastic sequences of roughly comparable age and thickness from the Los Angeles and Ventura basins.

Considerable discussion involves the two primary methods of defining the Pliocene-Pleistocene boundary--Reboul's 1833 concept implying evolutionary changes in fossil faunas gives way to Forbes' 1846 concept of paleoclimatic change reflected by fossil faunas. The writer herein places the boundary 4,000 ft (1,220 m) lower than that cited by Ogle (1953), or Faustman (1964).

Paleoecologic interpretations and basin analyses rely on faunal variability and faunal dominance in both foraminiferal and molluscan assemblages. Sediment textures generally reflect basin filling and westward migration of the Humboldt basin shoreline. Turbidite sequences appear to be important, especially in sections containing bathyal assemblages. Upwelling possibly influenced the distribution of some key foraminiferal faunules in northern California.

More than 180 species of forams are documented in the systematics and distribution charts, and two species are new and previously undescribed. Important questions are raised concerning the nomenclature of Bulimina subacuminata, Elphidium hughesi, and other species. The use of the generic term Islandiella for certain species of Cassidulina is also questioned. The Neogene bolivinids, mainly Bolivina, Bolivinoides, and Brizalina, are regarded as the most promising for future studies on phylogeny through the extensive use of SEM pictures.