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The oldest marine beds examined may be correlated with the upper Oligocene Sooke formation of Vancouver Island, British Columbia. All other marine beds examined in the area are younger than the widespread middle and upper Miocene Comondú formation which is involved in the formation of the Peninsula of Lower California. Post-Comondú marine beds are referred to the Pliocene and Pleistocene. The boundary between Pliocene and Pleistocene has been chosen rather arbitrarily on paleontologic grounds. The Pliocene is divided into three formations. Each has characteristic but closely related faunas. The middle formation (Carmen) may be correlated with the San Diego formation as exposed at Pacific Beach, which is accepted as of middle Pliocene age. Because of its close relationship to the Carmen formation, the lower formation (San Marcos) is considered lower Pliocene.

The San Marcos formation is correlated with the Imperial formation of southeastern California, which is therefore considered lower Pliocene. Evidence presented shows that the Imperial formation is the basal member of a series of recurrent late Tertiary and Pleistocene marine invasions of southeastern California.

A total of 273 species of pelecypods, gastropods, echinoids, corals, and barnacles have been examined and identified; of these 67 are presumed extinct. Forty-three species and subspecies are recorded or described as new. Pelecypods and gastropods constitute most of the fauna, although echinoids of the genus Encope are very important. Considerable selective leaching of fossils has taken place in the older Pliocene rocks, and consequently the faunas collected and recorded from those rocks are not representative. Examination of data on chemical composition of shells and tests shows that those containing the greatest amount of MgCO3 resist leaching the longest.

The Gulf of California faunal “province” is discussed, and physical data on the present Gulf are given. Of approximately 100 species each of living gastropods and pelecypods studied in the Gulf of California fauna, 10 and 30 per cent respectively are also living on the Pacific Coast at San Diego. This furnishes a standard for comparison and correlation of fossil faunas from the Gulf of California with those of the Pacific Coast of California.

From a limited survey of the ecology of living representatives of the fossil faunas, it is apparent that most of the latter lived in shallow waters, many of them in depths of less than 15 meters. The paleo-distribution of the reef coral genera Pocillopora and Psammocora (Stephanaria) indicates that at one or more intervals during the Pleistocene the temperatures were warmer in the Gulf than at present.

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