156 AXEL A. OLSSON 



group of monomyarian bivalves, most plentiful and often growing much 

 larger in cool or temperate waters. Their geologic history is long, extending 

 back into the Paleozoic. The Pectens offer special difficulties in their classi- 

 fication and more than a ICX) generic and subgeneric names have been 

 proposed for them but complete intergradations seem to occur between all 

 the principal groups. In this work, the Panamic species will be distributed 

 as follows. 



Key to Genera and Subgenera 



I. Shell with very unequal valves, the lower or right valve being inflated 

 or convex and with an incurved beak, while the left valve is nearly 

 or quite flat, depressed or concave and fits closely inside the margin 

 of the right valve when shut. 



Genus Pecten 



1. Subgenus Euvola. Relatively thin, the surface of the right valve nearly 

 smooth or with low, rounded ribs separated by narrowly grooved or 

 lined interspaces, the upper or left valve more heavily sculptured. Dis- 

 tribution, Caribbean, and West Atlantic; absent from the Panamic- 

 Pacific region. 



2. Subgenus Pecten, sensu stricto. Shell heavier, both valves strongly 

 ribbed and generally with longitudinally corded or striated sub- 

 sidiaries. 



3. Subgenus Flabellipecten. Similar to Pecten, s.s., but the right valve 

 is typically less convex, and the ribs are flatter, crossed mainly by con- 

 centric subsidiaries. 



II. Shell with the valves more or less equal in shape and convexity and 



if any difference exists, the upper or left valve is the more convex. 

 A. Shell with an external sculpture of strong, radial ribs. 



a. Shell often quite large, the ribs elevated, rounded, the surface of both 

 ribs and interspaces coarsely longitudinally corded or striated. 



Genus Lyropecten 



4. Subgenus Nodipecten. Umbonal surface of valves often more or less 

 undulated, the ribs strong, often humped or noded. 



b. Large or small Pectens with nearly equal valves, the ribs low, rounded 

 or squarish and overrun by relatively fine subsidiaries. Ears are sub- 

 equal in size, that of the right anterior bearing a byssal notch. 



Genus Aequipecten 



5. Subgenus Aequipecten, ss. Generally large or medium-sized shell, the 

 ribs and interspaces more or less roughened by a scabrous sculpture. 



6. Subgenus Plagioctenium. Medium to large shells, the sculpture formed 

 by squarish ribs and a series of subsidiary concentric threads looped 

 across the ribs and their interspaces. 



7. Subgenus Leptopecten. Kelp Pectens. Small, thin-textured shells, usually 

 with wide, unsymmetrical valves, squarish ribs and subsidiary con- 

 centrics. 



8. Subgenus Pacipecten. Fairly strong textured Pectens with circular 

 slightly convex, symmetrical valves with numerous rounded or flat- 

 topped ribs. Valves unlike in color, the right valve white, the other 

 dark. 



c. Large or medium-sized Pectens with high and narrow valves and unequal 

 ears, the anterior one much the larger. 



Genus Chlatnys 



