Inadequate reliable information on the Chico formation of the California Cretaceous led the writers to re-examine the type locality. Evidence is presented to show that Chico Creek must be considered to be the type locality, and an historical review of previous work is given. The Cretaceous rocks exposed in Chico Creek comprise a series of clastics 2000 feet in thickness, resting unconformably on the older Sierra Nevada metamorphics and covered at the top by Tertiary volcanics. The fauna of the Chico beds exposed in Chico Creek is abundant, but only meager evidence exists to correlate it with other Cretaceous localities in the central valley of California. Correlation with other more removed sections is difficult, and a conclusive tie into the European section cannot be made at this time. Typical fossils found in Chico Creek are discussed, and neotypes are illustrated for certain important species, the originals of which have been lost (Ammonites chicoensis Trask and Baculites chicoensis Trask). All figured specimens are deposited in the California Academy of Sciences.

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