PANAMIC-PACIFIC PELECYPODA 75 



3. Shell narrowly elongated, pholadiform, rounded in cross-section, the 

 anterior side much drawn out and wedged-shaped at the end, the 

 posterior side greatly shortened and carinated. 



Genus Litharca 



B. The ligament with the resilium (generally with grooves) does not 

 cover the whole surface of the cardinal area. 



c. Cardinal area relatively narrow, the ligament with the resilium 

 developed mainly over its posterior portion; the anterior portion 

 short, without transverse grooves, bare or covered by the periostra- 

 cum only. The radial and concentric riblets finely or coarselj'^ beaded 

 by their intersection. 



ca. The posterior-umbonal slope is strongly angled. 



4. Sculpture more or less uniform and similar over the whole surface. 

 Periostracum thin or lacking, white. 



Genus Acar 



5. Radial riblets very unequal in size, those on the posterior-umbonal 

 slope, large, highly elevated and coarse, the others low and flat. 



Genus Calloarca 



cb. The posterior-umbonal slope is low and rounded. 



6. Shell relatively thin, white, under a coarse, pilose, brown perio- 

 stracum, the radial riblets fine and numerous. 



Genus Fugleria 



d. The cardinal area relatively high with the ligament restricted to 

 a small, inverted V-shaped area under the beak; the ligament area 

 is vertically grooved or striated. 



7. Shell small, solid, convex, boat-shaped, the sculpture formed by fine 

 beaded, radial riblets. 



Genus Arcopsis 



II. Anadarinae. The shell is usually free in the adult stage or if held 

 by a short byssus, the valves show no byssal gap along the ventral 

 margin. The beaks are prosogyrous, facing forward over a cardinal 

 area of variable size. The ligament is mainly developed behind or 

 posterior of the beak, its area smooth or with transverse tent-shaped 

 grooves originating under the tip of the beak. The external sculp- 

 ture is typically formed by strong radial ribs which produce deep 

 flutings along the inner side of the ventral margin. 



8. A cardinal area is well developed, often quite high, and generally 

 equally formed on both sides of the beak. 



Genus Anadara 



9. The cardinal area is narrow and almost lacking so that the beaks 

 are closely adjacent to the hinge margin. Ligament largely restricted 

 to the posterior side, the posterior set of teeth hence the longest. 



Genus Lunarca 



III. Noetinae. Shell for the most part free. Beaks opisthogyrate or coiled 

 backwards above the cardinal area. Ligament and cardinal area 

 mostly or wholly anterior in position, vertically grooved or striated. 

 Surface sculptured by rounded or flattened ribs, the interspaces often 

 with interstitials. Posterior adductor scar bordered by a strong flange. 



