112 AXEL A. OLSSON 



ab. Shell partly modioliform, the beaks not quite terminal. Anterior side 

 very short, externally outlined by a faint lunular line, its inner margin 

 smooth or weakly toothed. 



5. Shell elongately wedge-shaped, dark brown to black, glossy and with 

 a satiny sheen, internally white or vinaceous. 



Genus Semimytilus 



ac. Shell modioliform, the anterior margin short, projecting but without 

 a lunular line, no hinge teeth. 



6. Shell elongate to hatchet-shaped, the ventral side often deeply im- 

 pressed. Periostracum smooth, scaly or bearing bristles. 



Genus Modiolus 



b. Borers in rock, coral or shell. 



7. Surface smooth or with concentric lines of growth, the periostracum 

 dark brown to black, and without secondary lime incrustation. 



Genus Botula 



8. Surface incrusted with a secondary coating of lime (except Lithophaga, 

 sensu stricto). Shell typically elongated, cylindrical or bullet-shaped. 



Genus Lithophaga several subgenera 



B. Surface sculptured with radial riblets, strong or partly obsolete, the 

 inner margins crenulated by the external ribbing at least in some 

 groups. Dysodont teeth present in most cases. 



c. Shell ovate to obliquely subcircular, the longer axis nearly vertical. 

 Sculpture with small divaricating riblets. 



9. Shell small, subnacreous, the hinge teeth strong. 



Genus Crenella 



d. Shell elongate, mytiliform to modioliform. 

 da. Individual, free or nestlers. 



daa. Cavity of the beak bearing a small, shelflike platform or septum on 

 which the anterior adductor scar is placed. 



10. Shell generally small, mytiliform, with ribbed sculpture, often with 

 lines of bristles. 



Genus Septijer 



dab. Cavity of the beak without a septum, the anterior adductor scar placed 

 on the shell wall itself. 



daba. Ligament relatively short, only about half the length of the hinge 

 line. 



11. Margins of the shell strongly crenulated all around. Dysodont teeth 

 strong along the anterior margin. 



Genus Brachidontes 



dabb. Ligament as long as the hinge line or nearly so. 

 dabba. Ligament extending forward to the tip of the beak. 



