PANAMIC-PACIFIC PELECYPODA 291 



Bb. The escutcheon is small, restricted to the left valve or wholly absent. 

 Hinge has the anterior cardinal tooth in the left valve, high and 

 elongated parallel to the lunular margin (resembles a lateral tooth) 

 while the basal arm of the central cardinal tooth in the right valve is 

 narrowly prolonged so as to connect with the lower rim of the anterior 

 socket. 



Bba. Shell a nestler or rock borer. 



8. Valves irregular and distorted. External sculpture is formed by widely 

 spaced concentric lamellae with smaller radials in the interspaces. 



Genus Paphonotia 

 Ebb. Shell free, its valves not distorted. 



Bbba. Lunule large and cordate in shape, defined by an incised hne. 

 Escutcheon small and restricted to the left valve. 



Genus Nioche 



9. External sculpture formed principally by scabrous or rasplike radial 

 riblets. 



Subgenus Nioche, s.s. 



10. Shell rounded and convex, the external sculpture cancellated {Antigona- 

 like). 



Subgenus Antinioche 

 Bbbb. Lunule and escutcheon both absent. 



11. Shell subquadrate, oblique. Color white and generally chalky. 



Genus Colonche 



11. Hinge with a small, sometimes nearly obsolete anterior lateral tooth in 

 the left valve (placed close to the base of the cardinal socket) and with 

 or without a lateral socket or pit in the right valve. Pedal muscle scar, 

 below the anterior cardinal tooth, is deeply sunken into the hinge plate. 



C. Shell oblong or rounded, convex, often solid. Surface strongly sculptured 



with coarse concentric ridges or lamellae. 

 c. Anterior-lateral tooth of the left valve strong, its socket or pit in the 



right valve usually visible. 



Genus Antigona 



12. Interspaces between the concentric lamellae marked with strong radial 

 cords or lines. 



Subgenus Antigona, s.s. 

 Not regional. 



13. Interspaces between the concentric lamellae marked only with fine 

 concentric threads. 



Subgenus V entricolaria 

 ca. Anterior-lateral tooth of left valve small or subobsolete. 



14. Sculpture formed by strong, reflexed concentric lamellae, crenulated 

 and with strong radial cords in their interspace. 



Genus Periglypta 

 Genus AJfTIGONA Schumacher, 1817 



Type species by monotypy, Antigona lameUaris Schumacher (Dosinia 

 lamarckii Gray, 1838). Indo-Pacific. 



Shell of medium size, subtrigonal to elongate subovate in form, the 

 posterior end truncate. Lunule large, deep, limited by an incised line. 

 Escutcheon excavated, narrow, and long. Valves convex. Each valve has 

 three cardinal teeth, the right posterior and the left middle teeth bifid and 



