Rocks of Late Cretaceous age osccur on the Pacific Coast of North America from Baja California, Mexico, to the Queen Charlotte Islands of British Columbia. The standard term Upper Cretaceous series is adopted for these deposits instead of the often widely and variously misused term Chico series (or “group” or “formation”), and Chico is applied only to those sediments on the east side of the Sacramento valley from whose occurrence on Chico Creek the term was derived. The series is best represented in the Sacramento and San Joaquin valleys of California where rocks of Cenomanian to Maestrichtian ages are recognized and thicknesses of as much as 28,000 feet have been reported. South of the Transverse Ranges of California only post-Cenomanian ages are represented, and over much of the area only Campanian-Maestrichtian. The Oregon-Washington-British Columbia sequences are similar in age to those of Baja California, but the Campanian-Maestrichtian is comparatively restricted in distribution. In neither the northern nor southern areas are the thicknesses comparable to those of the central area, but the sediments in all three consist primarily of sandstones, shales, and conglomerates, with limestone present only as concretions or local lenses. Throughout the Pacific Coast region the Upper Cretaceous appears to rest unconformably on various Lower Cretaceous or older rocks. The series is subdivided into the lower Pacheco group and the overlying Asuncion group, in many places separated by an unconformity. Subdivisions of these groups, and particularly the upper, are recognized over wide areas. Five hundred and four species of invertebrates are recorded. These include: 1 coral; 2 (1 new) crinoids (2 genera); 7 (2 new) echinoid 151 (45 new) pelecypod species (45 genera); 109 (28 new) gastropod species (40 genera); 223 (118 new) ammonite species (53 genera); 3 (2 new) nautiloid species (one genus); and 1 belemnite. The occurrence of 2 mosasaurs, 1 tylosaur, and 2 dinosaurs is also recorded. The following new genera and subgenera are proposed: Crinoidea, Pachecocrinus; Ammonidea, Neocyrtochilus, Extcrioceras, Neokotôceras, Joaquinites, Oregoniceras, Butticeras, and Eocanadoceras (subgenus).