H AXEL A. QLSSON 



19. Shell wedge-shaped, the posterior side shorter, often flattened 

 and with subtruncated margin, the texture solid. Surface smooth 

 and pohshed, usually showing fine radials which crenulate the 

 margins. Mostly beach shells. 



Donacidae 



20. Shell elongated, razor-shaped, the beaks submedian or close to the 

 anterior end. Surface with growth line sculpture, sometimes cut 

 by oblique lines. 



Solenidae 



21. Mostly rock borers with subovate to subelongate valves, often 

 deformed. Surface with ribbed or concentric sculpture, sometimes 

 with zigzag markings. Hinge similar to the Veneridae. 



Petricolidae 

 Dbbab. Pallial line entire, no sinal inflexion. 

 Dbbaba. Shell permanently attached to the substratum. Hinge with large, 

 irregular-shaped teeth, 



22. Shell solid, porcellaneous, generally irregular in shape, attached 

 by the umbone of either valve. Sculpture formed by ribs and con- 

 centric lamellae. Generally strongly colored. 



Chamidae 



Dbbabb. Shell free not attached at any time. 



Dbbabba. Mostly fresh-water or brackish species, the surface covered by 

 a thick, coarse periostracum, generally destroyed over the beaks 

 and umbones which are deeply corroded and chalky. 



23. Shell generally cordate or rounded, equivalve, with anterior beaks 

 and umbones, the hinge provided with both cardinal and lateral 

 teeth. Pallial line mostly entire, sometimes with a small posterior 

 sinus. Color white, often with pink or purple shading. 



Corbiculidae 



Dbbabbb. Marine species, the surface of the umbones and beaks generally 

 not corroded. 



24. Shell cordate, veneriform, equivalve with prominent umbones and 

 recurved, prosogyrate beaks, solid. Surface often with ribbed 

 sculpture and fluted margins. 



Carditidae 



25. Shell subovate, cordate, convex to depressed. The dorsal areas 

 are frequently set-off by differences in contour and sculpture. 

 Hinge teeth variable, often become obsolete. The anterior ad- 

 ductor scar is generally long and narrow, lying within the cir- 

 cumference of the pallial line. 



Lucinidae 



26. Shell similar to the Lucinidae in shape but generally smaller and 

 thinner. Cardinal teeth conspicuously double or bifid. Adductor 

 scars of equal size. 



Diplodontidae 



