PANAMIC-PACIFIC PELECYPODA 45 



Dab. Cardinal area well developed. Substance of shell porcellaneous, 

 the surface with plain or strong sculpture. With or without an 

 anchoring byssus. 

 Daba. Shell inequilateral and often inequivalve. Hinge line straight. 



12. Surface with ribbed sculpture, often unequally developed in the 

 two valves. 



Arcidae 

 Dabb. Valves nearly symmetrical, the sides subequal. Hinge line arched 

 or curved. 



13. Ligament spread over the greater part of the cardinal area. 



Glycymeridae 



14. Most of the cardinal area bare, the small ligament internal and 

 lodged in a triangular pit or resilifer under the beak. 



Limopsidae 



Db. Hinge concentrated, usually composed of a few, strong teeth which 

 fit into sockets. 

 Dba. Hinge torsional, the teeth hook-shaped, curving outward from be- 

 low the beaks. Cyclodonta. 



15. Shell cordate or heart-shaped, convex, the valves equal. Surface 

 smooth or ribbed. 



Cardiidae 



Dbb. Hinge not torsional, the teeth seated on a platform along the 

 margin. The teeth below the beak are called cardinals; those 

 spaced along the margins are known as laterals. Teleodonta. 

 Dbba. The ligament is external, usually placed behind the beaks. 

 Dbbaa. Interior of the shell showing a deep pallial sinus. 



16. Shell cordate or heart-shaped, the valves alike in sculpture and 

 convexity, the umbones usually prominent, ending in prosogyrate 

 beaks, the anterior side usually shorter. 



Veneridae 



17. Shell lenticulate, elongated, thinner and generally inequivalve, 

 the umbones flattened, inconspicuous, the posterior end of shell 

 shorter, flexed, and sometimes rostrated. Surface smooth, with 

 concentric growth incrementals, more rarely radially ribbed. Pallial 

 sinus deep, the margin of the valves usually smooth. 



Tellinidae 



18. Similar to the last but the shell often razor-shaped, thin or of 

 medium weight, generally equivalved, gaping posteriorly (except 

 Heterodonax). Color white, pink, or maculated and rayed with 

 purple, the periostracum coarse and thick. Ligament external, at- 

 tached to a stout nymphal plate which rises prominently behind 

 the beaks. 



Sanguinolariidae 



