PANAMIC-PACIFIC PELECYPODA 47 



Dbbb. Ligament almost wholly internal and divided into two unequal 

 parts, the larger section or resilium lodged in a chondrophore 

 (resilifer) or an excavated, cup-shaped pit in the hinge plate 

 between the hinge teeth or carried on a projecting arm, 

 Dbbba. Pallial line entire, no posterior sinus. 



27. Shell elongated or subrectangular, solid, the posterior side often 

 pointed, rostrated. 



Crassatellidae 

 Dbbbb. Pallial sinus strong. 



28. Shell ovate or trigonal with high, full umbones and prosogyrate 

 beaks, thin or solid, the color generally white. Resilifer is a deep, 

 cup-shaped chondrophore in the middle of the hinge plate between 

 the cardinal teeth. Posterior-dorsal area usually well defined by 

 an angle, ridge or elevated frill. Valve margins smooth. 



Mactridae 



29. Shell subcircular to subelliptical, convex or depressed, the posterior 

 side sometimes flexed. Surface smooth or with elaborate sculpture 

 of ribs, lamellae. Pallial sinus large and deep. Resilifer an oblique 

 groove in the hinge plate, similar in both valves. 



Semelidae 

 Dc. Hinge structure relatively simple, the teeth small, irregular in shape 



or wholly wanting. 

 Dca. Substance of the shell porcellaneous, the ligament either external 

 or internal; if internal, the resilium is attached to a spoon-shaped 

 chondrophoral arm often simulating a large tooth which projects 

 outward from the hinge plate in the left valve (asthenodont). 

 Nestlers, rock borers or living buried in sand or mud. 

 Dcaa. Ligament external. 



30. Species with both large or small shells, the valves gape strongly 

 behind, the surface sculptured with crude or wrinkled concentric 

 lines, occasionally rayed. Borers or nestlers in rock, shell, or coral. 



Saxicavidae 



Dcab. Ligament internal. Resilifer carried on a chondrophoral arm in the 



left valve which fits into a wide gap in the right valve, with the 



resilifer scar within, under the umbone. 



3L Valves more or less equal in size, shape and convexity, the umbones 



rather low. PaUial sinus usually large and deep. 



Myacidae 

 32. Shell usually small, the valves alike or unequal in size, often 

 rostrated or pointed posteriorly. Chondrophoral plate resembles 

 a cardinal tooth. 



Corbulidae 



Deb. Shell typically nacreous, rarely subporcellaneous, the surface layer 



often earthy, chalky, or granulose. Ligament internal, the resilium 



lodged in a chondrophore which is similar in both valves. 



( Anomalodesmacea ) . 



