574 



MOLLUSCA 



PINNA L. Shell equivalve, wedge-shaped, with the pointed end, on 

 which is the umbo, anterior in position ; posterior end broad and gaping ; 

 hinge teeth absent: 30 species, some of which produce black pearls of 

 considerable value. ' 



P. muricata L. (Fig. 903). Shell white and semitransparent, with 

 straight sides with obscure longitudinal ribs, which are covered with 

 scales; length 17 cm.: North Carolina to Texas. 



FAMILY 2. OSTREIDAE. 



Oysters. Shell inequivalve, resting on and attached by the left 

 valve, irregular and variable in shape, very thick, often in folded layers; 

 foot absent: 1 genus. 



OSTREA* L. Shell circular or elongate, upper valve more or less flat 

 and the lower convex; umbo and hinge at forward end; hermaphroditic 

 or unisexual; adductor muscle near the middle with the heart just in 



Fig. 903 



Fig. 904 



Fig. 903 Pinna muricata (Rogers). Fig. 904 Ostrea virginica left valve 

 removed to show organs (Brooks). 1, right valve of shell; 2, mantle; 3, gills; 

 4, labial palps ; 5, rectractor muscle ; 6, mouth ; 7, intestine ; 8, stomach sur- 

 rounded by the liver ; 9, anus ; 10, visceral mass ; 11, posterior end of animal ; 

 12, dorsal side of animal. 



front of it; muscle impressive violet or brown; shell white: about 100 

 species, in all seas except the colder ones ; 1 species on the Atlantic and 

 1 on the Pacific coast; 500 fossil species. 



0. virginica Gmelin (Fig. 904). The American oyster. Length up 

 to 45 cm., usually about 12 cm. : Gulf of Mexico to Massachusetts, locally 

 to Gulf of St. Lawrence, in shallow and brackish water, often very 

 abundant, the most valuable American invertebrate animal; introduced 

 on the Pacific coast. 



* See "The Oyster," by W. K. Brooks, 2nd Ed., 1905. "A Manual of Oyster 

 Culture," by Caswell Grave, Fourth Rep. Maryl. Shellf. Com., 1912. 



