Prodissoconch with long hinge; two denticles 

 at each end, the anterior pair frequently reduced; 

 the ligament is internal at the level of the hinge, 

 at the center, and between the center and anterior 

 end (Ranson, 1960, fig. 350, ch. XVI). 



Genus Crassostrea Sacco, 1897. Genotype: C. virginica 

 Gmelin 



Shell very variable, usually elongated; lower 

 valve cuplike, deep, and recessed under the hinge; 

 upper valve flat, opercular. Muscle scar dis- 

 placed in dorsolateral direction. Large promyal 

 chamber on the right side of the body. Gill 

 ostia and eggs relatively small. Nonincubatory. 

 Excellent illustrations of the species can be found 

 in the monographs of Lister (1685) and Chemnitz 

 (1785). 



This genus includes species formerly known as 

 0. virginica, 0. gigas, and G. angulata. The 

 separation of the cuplike oysters (Crassostrea) 

 from the flat ones (Ostrea) is justified because of 

 the anatomical differences (promyal chamber, 

 size of the ostia) and spawning habits. The name 

 "Crassostrea" (Sacco, 1897) is validated in ac- 

 cordance with the rules of the International Com- 

 mission on Zoological Nomenclature (1955). 



Valves of the prodissoconch unequal; hinge with 

 two teeth at each end; internal anterior ligament 

 extends beyond the hmge (Ranson, 1960). (Fig. 

 348, ch. XVI). 



Genus Pynodonte Fisher de Waldheim, 1835. Genotype : 

 P. radiata F. de W. 



Shells large and heavy; lower valve slightly 

 recessed under hinge; both shells lack sculpture 

 except for sharp crenulations along the tip; hinge 

 very broad. Inner sides of valves chalky white 

 or greenish; row of small denticles along the edges 

 of valves on both sides of the hinge; muscle scar 

 white, elevated on a shelf like projection; the ad- 

 ductor muscle is oval in outline and rounded on 

 the hinge side; the gonads are bright orange; the 

 ventricle of the heart surrounds the rectum, which 

 runs posteriorly beyond the adductor muscle 

 almost to the junction of the mantle. Non- 

 incubatory. 



Valves of prodissoconch equal; hinge with five 

 teeth arranged over the entire length of it; internal 

 anterior ligament, immediately after the hinge; 

 10 small denticles at the edge of each valve an- 

 teriorly to the ligament (Ranson, 1960). 



The following species of living oysters are known 

 from the coastal waters of the continental United 

 States and from the State of Hawaii: 



C. virginica Gmelin, Eastern oyster, Atlantic 

 oyster. This is the principal edible oyster of the 

 Atlantic and Gulf Coasts of the U.S.A. (fig. 4). 

 Its range of distribution extends from the Gulf of 

 St. Lawrence to the Gulf of Mexico and the West 

 Indies. The species was introduced in the waters 

 of San Francisco Bay, Puget Sound, Willapa Bay, 

 and Oahu Island but failed to establish itself, 

 although occasionally single specimens can be 

 found in these waters. 



The right (upper) valve smaller than the left. 

 The beaks elongated and strongly curved. The 

 valve margins straight or only slightly undulating. 

 The muscle scar usually deeply pigmented. The 

 adductor muscle located asymmetrically, well 

 toward the posterodistal border. Large promyal 

 chamber on the right side. Nonincubatory, dis- 

 charging eggs and sperm directly into the water. 



Adults vary from 2 to 14 inches in height 

 (dorsoventral direction) depending on age and en- 

 vironment. Shape, sculpture, and pigmentation 

 of inner side of the shell and along the edges of the 

 mantle and tentacles vary greatly. 



Crassostrea rhizophorae Guilding. Light, thin, 

 foliaceous, and deeply cupped shell with smaller 

 flat upper valve fitting to the lower one (fig. 5). 

 The inner margins straight and smooth with con- 

 siderable purple coloration especially around the 

 left valve. The beaks twisted dorsally. Muscle 

 scar near the dorsal margin. Promyal chamber 

 present. Nonincubatory, discharging eggs and 

 sperm directly into the water. 



Similar to C. virginica from which it differs by 

 the following characters: lower left valve is less 

 plicated than in virginica; the muscle scar is more 

 rounded and often unpigmented. (Prodissoconch 

 shown in fig. 349, ch. XVI). 



Adults may reach 4 inches in height. Fre- 

 quently attached to the aerial roots of the man- 

 grove Rhizophora mangle. Inhabits Caribbean 

 region including Puerto Rico and Cuba where the 

 species is commercially exploited. 



Crassostrea gigas Thunberg, Japanese oyster, 

 Pacific oyster (fig. 6) Cuplike shells of large size 

 with coarse and widely spaced concentric lamellae 

 and coarse ridges on the outside; shells usually 

 much thinner than those of C. virginica. Upper 

 (right) valve flat and smaller than lower (left) 



TAXONOMY 



