PANAMIC-PACIFIC PELECYPODA 359 



Valves elliptical, orbicular to nearly circular, generally subequal, or if 

 unequal, as in some species, the right valve is a little larger and more con- 

 vex than the left. Beaks prosogyrate, nearly median or placed a little behind 

 the middle. Ligament mainly internal, the resilium seated in a deep, elon- 

 gated chondrophore, similar in each valve, and separated from the much 

 smaller tensilium or external part of the ligament by a wide, nymphal 

 plate. Hinge with the cardinal teeth small, sometimes partly obsolete, the 

 lateral teeth much stronger, equidistant. The pallial sinus is large and deep, 

 rounded at the end, free from the palhal line below it. Surface sculpture 

 varies from nearly smooth shells or with concentric growth lines only, to 

 highly sculptured forms with strong ribs and concentric lamellae, some- 

 times a combination of both. 



Key to Panamic-Pacific species of Semele 



I. Valves are of unequal size, the left valve decidely larger and more 

 convex. 



1. Shell large (90 mm., or more), the sculpture formed by growth lines 

 and weak radial striae. External color white, the interior apricot. 

 Peruvian. 



S. corrugata 

 II. Valves are nearly equal in size, the external surface plain, smooth or 

 strongly sculptured. 



A. Surface smooth or marked with growth line concentrics only. 

 Aa. General size large (40 mm. or more). 



a. An escutcheon or flattened area along the posterior dorsal margin is 

 absent. 



aa. Shell broadly subovate to elliptical in shape. 



2. Anterior side much longer than the posterior, the surface smooth and 

 polished. 



S. la e vis 



3. Shell higher, subovate, the posterior weakly flexed. Surface marked 

 with fine, incised, concentric lines (dosinoid) and weak, radial striae. 



S. elliptic a 



b. An escutcheon is present, the posterior-dorsal slope strongly flattened 

 and bordered by an angle. 



4. The shell is subcircular in shape, its surface white and covered by a 

 thin, gray or yellow periostracum. The concentric sculpture is irregularly 

 developed, overrun by fine, radial lines, often granulose on the sides. 



S. flavescens 

 Ab. Shell smaller (40 mm. or less). 



5. Shell elliptical in shape, the anterior side much longer than the pos- 

 terior. Sculpture consists of fine, concentric lines intersected by oblique 

 lines on the anterior side. Color white, mottled with yellow, violet or 

 brown. 



S. sparsUineata 



6. Shape subcircular to subelliptical, nearly equilateral. Surface marked 

 with fine concentrics, regularly distributed and minute or submicro- 

 scopic striae fanning out on the lateral slopes. Color usually white, the 

 umbones and interior stained with yellow, purple or brown. 



S. lenticular e 



B. Sculpture stronger. 



Ba. Sculptural elements predominantly concentric, if any radials are present, 

 they consist of fine striae only. 



