8 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



8. Shell (right valve) attached to foreign object (by 

 cementation) ; usually distorted and irregular in out- 

 line; right valve deep, left flatter, both quite heavy; 

 byssal sinus obsolete; ribbing coarse and irregular 



Genus Hinnites (adult) 



8. Shell not attached to foreign object 



9. Both valves quite convex (right deeper than 

 left) ; moderately thick; 18-21 ribs (except ir- 

 radians amplicostata\ 12-15) ; concentric lamellae 

 in many species, often u^eak or absent in adult 



stage; no spinose sculpture 



. . . . Subgenus Argopecten 



9. Both valves quite flat (right deeper than left) ; 



rather thin; 18-22 ribs, broad and flat on right 



valve, narrow and rounded on left; right valve 



with fine concentric growth lines, left with very 



fine concentric lamellae 



Subgenus Patinopecten 



9. Both valves moderately convex 



10. Shell translucent or subtranslucent ; more 



than 30 plicate ribs 



Subgenus Juxtamusium 



10. Shell moderately thick (except very thin 

 (Leptopecten) latiaurata monotimeris) ; 5- 

 100 or more ribs and/or riblets 

 11. 17 or more ribs; left valve deeper than 

 right ; spinose or imbricate sculpture usu- 

 ally present but no radial striae or con- 

 centric lamellae 



. . Gtnns Chlainys; ]\XYtni\Q Hinnites 

 11. 5-16 ribs, always plicate; right valve 

 deeper than left 



12. Hinge line very long, longer than 

 disk, same length, or slightly 

 shorter; orbicular or nearly; 9-16 

 ribs . . subgenus Leptopecten 

 12. Hinge line moderately long, more 

 than y2 but less than % length of 

 disk; usually higher than long; 9- 

 13 ribs . Genus Semipallium 



