PANAMIC-PACIFIC PELECYPODA 377 



5. Valves smaller, unequal, usually with strong sculpture. Posterior end 

 pointed, or produced into a short beak. 



Genus Mensca 



b. No differentiated posterior-dorsal area. 



6. Valves trigonal, unequal, the dorsal margin serrated or toothed. Surface 

 marked with wavelike, concentric undulations. 



Genus Tellidora 

 AD. Valves rounded, subequal, convex or depressed. 



7. Sculpture formed by oblique and generally divaricated or sharply 

 flexed lines. 



Genus Strigilla 



B. Right valve with the anterior lateral tooth placed near or against the 

 cardinal tooth. 



BA. The ligament is wholly external, attached to a narrow, cross-striated 

 scar. 



c. A posterior-dorsal area present, defined by its impressed surface, 

 stronger sculpture and bordered by an angle or line. 



ca. Shell elongately subquadrate or subelliptical in shape, compressed or 

 slightly convex, the anterior side longer and more rounded, the posterior 

 side sloping and somewhat flexed. 



8. Shell usually large, (generally over 25 mm. in length), surface smooth 

 or marked with evenly spaced incised Hnes or concentric sulci. 



Genus Eurytellina 



9. Similar but with a fine, reticulate sculpture. 



Genus Tellinidella 



cb. Shell small, usually less than 25 mm. in length. 



10. Surface smooth or with fine, concentric lines, or, as in the subgenus, 

 Scissula, with fine, incised lines cutting across the concentrics obliquely. 



Genus Moerella 



11. Shell subtrigonal to subovate, thin, subequivalve, the anterior side 

 expanded and convex, the posterior side shorter, sloping and pointed. 

 Surface smooth. 



Genus Elpidollina 



d. Posterior-dorsal area not well defined. Shell subquadrate in shape, 

 Sanguinolaria-Vike. 



12. Surface relatively smooth but generally with faint, obhque, incised 

 lines across the middle. 



Genus Hertellina 



BB. Ligament partly internal, the resilifer section short and partly im- 

 mersed in the hinge plate. 



13. Large or medium-sized, relatively thin. Color white with a yellowish 

 or ochraceous tinge. 



Genus Scrobictdina 

 IL Hinge tvith cardinal teeth only. Macominae 



C. Color a dull white, the surface often earthy or chalky, and covered in 

 life by a coarse, dark-colored periostracum. 



14. Shell subovate in shape, the posterior side shorter, pointed, and gen- 

 erally flexed. Northern. 



Genus Macoma 



D. Color a more brilliant white, the surface smooth and polished. Perio- 

 stracum thin and light in color. 



Da. Adductor scars of unequal size and shape, the anterior scar is longer 

 and narrower than the other. 



e. Valves with a deep, folded, posterior flexure forming a large, widely 

 expanded or winged posterior area. 



