PLIOCENE TO RECENT CHAMIDAE (BIVALVIA) FROM THE WESTERN NORTH ATLANTIC OCEAN

Eight Recent species can now be distinguished from the western North Atlantic with six species in the genus Chama and two species in the genus Pseudochama. The genus Arcinella occurs in the lower Pliocene to Recent in this region. Three of the Recent Chama species and one Pseudochama species occur in the Pliocene and Pleistocene fossil record. Additionally, six species of Chama and two species of Pseudochama are present only in the Pliocene and Pleistocene. Arcinella is attached to a firm substrate during early ontogeny, but Chama and Pseudochama remain permanently cemented to a hard substrate. During the Pliocene, Chama was dominant locally in the molluscan community and formed bioherms in Virginia and the Carolinas. The genus Pseudochama was proposed for species with the apex of the free left valve coiling in a clockwise spiral, but the genus name Chama has been used by some authors for taxa with clockwise or counterclockwise coiling. Characters for species descriptions include coiling direction, presence of inner margin crenulation, position of pallial line relative to muscle scars, maximum adult size, and foliated sculpture, but species are highly variable in shape, color, and sculpture. These morphological characters are being used to revise the systematics of fossil and Recent species. Also, the ITS1 and ITS2 DNA sequences are being examined in order to determine the relationships between Chama and Pseudochama, and to determine the validity of Pseudochama as a distinct genus.