PANAMIC-PACIFIC PELECYPODA 157 



9. Subgenus Chlamys, s.s Ribs small, numerous, sometimes cordlike and 

 rendered scabrous by the intersection of strong longitudinals and con- 

 centrics. Mostly in cold waters. 



10. Subgenus Flexopecten. Small shells with relatively few, large, high ribs 

 as if the valves had been sharply folded. 



B. Shell without pronounced external sculpture, the surface often smooth 

 and polished. Valves thin. 



Genus Amusium 



11. Subgenus Amusium, s.s. Shell often large, thin, rounded and disklike, 

 gaping at both ends, both valves smooth, polished but with strong 

 internal simple or paired ribs. 



12. Subgenus Cyclopecten. Shell very small, thin, with unequal valves and 

 generally discrepant sculpture, the right valve often completely smooth, 

 the left with concentric and faint radials. No internal lirae. 



Genus PECTEN Miiller, 1776 

 Type species by subsequent designation, Schmidt, 1818, Ostrea maxima 

 Linne. 



Shell pectiniform, equilateral or nearly so, with median umbones and 

 beaks, inequivalve, the right valve convex, often deeply so, the left valve 

 flattened or depressed. The single adductor scar is large, rounded and placed 

 slightly posterior of the middle line. Surface sculptured with radial ribs, those 

 of the right valve larger and rounded, those of the left valve usually smaller, 

 narrower, and with wider interspaces; concentrics may also be present. 

 Ears subequal, the anterior ones without a strong byssal notch. 



Three species. 



Key to species of Pecten 



I. Right valve is deep and strongly convex. Ratio of diameter to height 

 about 1 to 3. 



1. Shell rather large (length 70 to 100 mm.). Ribs of the right valve 

 rounded, simple; of the left valve flattened, with rather narrow inter- 

 spaces. 



P. vogdesi 



2. Shell much smaller, thinner, generally less than 40 mm. in length. Ribs 

 of the right valve, more or less deeply grooved ventrally; of the left 

 valve, narrower with wider interspaces carrying an intercalary riblet. 



P. perulus 

 II. Right valve shallow, the convexity rather small. Ratio about 1 to 8. 



3. Shell large or of medium size (length 50 to 70 mm.). Ribs of right 

 valve become bi- or tri-carinate at the end; those of the left remain 

 simple. 



P. sericeus 



Subgenus PECTEN s.s. 



Shell large, widely ovate, the right valve moderately convex, the left 

 valve flat with arched submargins and deeply impressed umbone and beak. 

 Sculpture is formed by strong, rounded ribs between concave interspaces, 

 the whole surface overrun by fine corded radials, the intervals between 

 the radial cords further ornamented with the edges of close-set concentrics. 

 Ears are subequal in size without a defined byssal notch. Hinge bearing 

 strongly wrinkled crura. 



