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Bivalved mollusks of the family Arcidae have been present in abundance on the Pacific Slope of North America since late Cretaceous time, 85 species and subspecies and 9 genera being recognized. With few exceptions the living species are confined to shallow water, chiefly in and south of the Gulf of California, although a few live as far north as southern California. The fossil representatives, particularly of Cretaceous age, attained wider geographic distribution, extending along the entire west slope of North America at least as far north as British Columbia. Although only a few species are known from Cretaceous beds and none from Paleocene, the family is well represented by fossils in Eocene and younger sediments, particularly those of Miocene and Pliocene age. Forty-one species, nearly half the total number recognized, are known in the Recent fauna.

Many of the species and genera have restricted geologic ranges, and it is chiefly for the purpose of assisting in their identification and promoting their use as age indicators that the main body of this paper was written. This is a systematic catalogue of species, with descriptions and data on geologic and geographic occurrences, and references to the literature of each fossil species. Living species are also catalogued and discussed briefly. Illustrations are provided of each extinct fossil species recognized, and of many Recent ones, including at least one representative species of each recognized subgeneric group.

This paper is one of a series dealing with these mollusks. In an earlier paper, the classification of the Arcidae was dealt with. Other papers have described and discussed various species and genera. One, written jointly with H. G. Schenck, describes the Oligocene representatives of Anadara as known throughout the world. The classification adopted in these earlier papers is retained in the present one. The Pacific Slope Arcidae are divided into three subfamilies: Arcinae (including Arca s. s., Litharca, Barbatia s. s., Calloarca, Obliquarca, Cucullaearca, Acar, Fugleria, Arcopsis, Bathyarca s. s.), Anadarinae (including Anadara s. s., Larkinia, Cunearca, Scapharca, Lunarca) and Noetiinae (including Noetia s. s., Eontia, Sheldonella, Trigonarca, and Halonanus). Arca noae Linné is accepted as the genotype of Arca.

The following new specific names are introduced: Arca (Arca) hawleyi, n. sp., Anadara (Anadara) carrizoensis, n. sp., and Anadara (Anadara) topangaensis, n. sp. Some species names are rejected as synonyms.

A number of revisions are made in the generic allocation of the various species dealt with, and one species (Arca (Barbatia) cowlitzensis Weaver and Palmer, from the Eocene of Washington) is removed from the Arcidae and assigned to the parallelodontid genus Porterius.

New information is presented on the stratigraphie allocation of many of the species, and in particular some of the Miocene species have been tied in, by means of associated Foraminifera, with the stages proposed by R. M. Kleinpell.

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