MOLLUSCA — PELECYPODS 



225 



Ligament in a groove, partly 

 internal and partly external. 

 Only posterior adductor scar 

 present in adults. 



Meleagrina (Fig. 94), a 

 sub-genus of Pteria, is of 

 especial interest from the 

 fact that one of its species, 

 M. margaritifera, is the chief 

 source of the white pearls of 

 commerce. Pearl oysters 

 are found especially in the 

 East Indies. For the origin 

 of pearls see page. 220. 



Fig. 95. — The common oyster, Ostrea virginica 

 Gmelin, from the Atlantic coast of North 

 .\merica. A , longitudinal section through the 

 valves showing the opening ligament (/.), the 

 closing or adductor muscle {a.m.), the an- 

 terior progression of the muscle scar {m.s.), 

 which in this species is black, and the internal 

 thickening of the shell (/.) ; j., junction of the 

 two valves; /'., ligament area; l.v., left valve; 

 r.v., right valve. B, interior of right valve^ 

 (Both X |.) 



Fig. 94. — The "pearl oyster," Melea- 

 grina margaritifera, from tropical seas. 

 (Xi) 



1. Sketch exterior of 

 one valve, noting wing, 

 ear, byssal sinus. 



2. How are pearls 

 formed ? 



Ostrea (Fig. 95). 

 Fennsylvanian to present. 

 Shell irregular, at- 

 tached when young to 

 other objects by the left 

 or larger valve. Beaks 

 terminal. Left valve 

 convex, right valve flat 

 or concave. Structure 

 of shell lamellar. Ex- 

 ternal surface more or 

 less distinctly ribbed. 

 Teeth generally absent 

 but hinge line broken by 

 a triangular cavity for 

 the ligament. Anterior 



