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 



7 ^/ 



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. 



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

 to 45 cm., usually about 12 cm.: Gulfjof MexTxn^-teJktassachusetts, locally 

 to Oiilf 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, 2Dd Ed., 1905. "A Manual of Oyster 

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



