44 AXEL A. OLSSON 



4. Hinge line straight, the cardinal area grooved by a series of ver- 

 tical pits for the attachment of the ligament. 



Isognomonidae 



5. Hinge line straight, the cardinal area narrow, elongated and 

 simple, with a single resilifer under the beak. 



Pteriidae 



B. Shell free or attached only in the earlier stages of growth. Porcel- 

 laneous, never pearly. 



6. Valves almost bilaterally symmetrical, and nearly equilateral, 

 pectiniform, with well-developed anterior and posterior ears. Hinge 

 line straight, the resilifer a pit under the beak. Surface smooth or 

 with radial ribs; often brightly colored. 



Pectinidae 



7. Valves oblique, inequilateral and often gaping on the sides, the 

 lateral wings less perfectly developed. Color usually white, smooth 

 or with radial ribs, often scabrous. 



Limidae 



n. Dimyarian. Each valve has two adductor scars placed near the anterior 

 and posterior ends of the shell. 



C. The adductor scars are of unequal size (Anisomyarian), the anterior 

 one is always small and sometimes partly effaced by the advancing or 

 growing margin of the hinge area. Byssiferous. 



8. Shells usually growing in clusters, attached to rocks, piling. Valves 

 mytiliform or modioliform, the beaks small and placed at or close 

 to the anterior end, the surface plain or ribbed, white, brown or 

 bluish-black, covered with a smooth or bristly periostracum. In- 

 terior pearly. 



Mytilidae 



9. Shell elongate, narrowed, and pointed anteriorly, fan-shaped, 

 brown or horn-colored, flexible around the wide margins. Seden- 

 tary forms buried in sand or gravel, the shell held in a vertical 

 position by the byssus. 



Pinnidae 



D. Adductor scars are more or less alike in size and shape. 



Da. Hinge structure formed by numerous, small vertical or > — 

 shaped teeth, on one or both sides of the beak, continuous or in- 

 terrupted in the middle (taxodont). 

 Daa. No cardinal area. The ligament attached to a central resilifer 

 under the beak. 



10. Shell rounded or ovate (nuculiform). Interior pearly. Ligament 

 internal. 



Nuculidae 



11. Shell elongated, the posterior side narrowed and rostrated, por- 

 cellaneous. An external ligament is present. 



Nuculanidae 



