DIAGNOSIS OF THE GENUS CRASSOSTREA (BIVALVIA, OSTREIDAE)

The oyster genus Crassostrea is the only valid genus in the subfamily Crassostreinae. Characters or tendencies considered diagnostic of othe r crassostreine genera are either environmentally controlled or can be found in the type species, C. virginica, or in its direct forerunner, C. gigantissima. Late larval forms of this genus all possess distinct ive convexities and ligament placement; adults have a right side promy al passage and a nonorbicular adductor muscle scar. A large size and e longate outline, a cupped left valve, an umbonal cavity, posterior and /or ventral displacement of the muscle scar, large void chambers, and significant nonvesicular chalky deposits are skeletal characters that may be present in some populations or species of this extremely variab le taxon. Crassostrea as rediagnosed has few living species; the taxon is evolutionarily conservative. Applying this conservatism to the fos sil record, recognizing that chomata are a part of the history of the genus, and realizing that similar evolutionary changes have not been s ynchronous throughout the geographic range of the genus, are all essen tial to deciphering the geologic history and evolution of Crassostrea. Ongoing and future biological work should contribute significantly to the understanding of this history.