PANAMIC-PACIFIC PELECYPODA 97 



noded for the most part while those of the right valve have nodes only on 

 the anterior seventh or eighth, the others are smooth. Ribs number about 

 22 or 23, are rectangular in section, somewhat wider than their interspaces 

 which are often strongly cross-threaded. Periostracum thin, deciduous, light 

 brown or straw-colored with a row of strong, coarse bristles in the inter- 

 spaces. 



Length 20 mm., height 17.7 mm., diameter 15.1 mm. 



Common. This species resembles A. chemnitzi (Philippi) of the Carib- 

 bean but is generally smaller and with narrower and more oblong umbones. 



Range — Gulf of California to northern Peru. Panama: San Carlos. 

 Ecuador: Mompiche; Limones; Galeras; Manta; Montanita; Santa Elena. 

 Peru: Tumbez; Zorritos; Mancora; Lobitos; Negritos. 



Subgenus SCAPHARCA Gray, 1847 



Type species by original designation, Area inaequivalvis Bruguiere. 

 Recent. Coast of southeastern India. 



Shell of medium size, oblong to subrectangular, relatively thin, moder- 

 ately convex with a rounded umbonal slope, the umbones placed at the 

 anterior one-third, flattened or slightly sulcate. The valves are slightly 

 unequal, the left valve being a trifle larger, its margin overlapping that 

 of the right. Sculpture is similar in each valve and consists of smooth ribs, 

 rectangular in section and separated by flattened interspaces of the same 

 width, the interspaces interlaced by strong cross threads. The cardinal area 

 is elongate, narrow, marked with one or two ligamental grooves; ligament 

 covers the area except for a narrow marginal zone on the anterior side. 

 Teeth numerous, arranged in a continuous series, divergent at the ends. 



In the type species, the shell is relatively thin so that the ribs show 

 through into the interior as strong flutings. Sculpture is similar on both 

 valves. The ribs are smooth and separated by flattened interspaces. 



Subgenus SECTIARCA new subgenus 



Type species, Anadara floridana Conrad (^4. secticostata Reeve of some 

 American authors). Plate 8, figure 2. 



Shell elongately subrectangular, subequivalve, moderately convex and 

 heavy. Umbones wide, the beaks small, prosogyrous. Cardinal area elongate, 

 much longer than wide, the ligamental grooves mainly developed over the 

 posterior portion, the anterior portion wholly or partially bare. Ribs nu- 

 merous, narrower than their flat interspaces, typically lined or grooved in 

 each valve. Hinge long, narrow with small, numerous teeth in a continuous 

 series. Posterior side oblique but not winged or emarginated. 



Sectiarca is similar in shape to Scapharca but has a heavier shell and 

 the valves are alike, without an overlapping margin. Ribs are typically 

 grooved in each valve. Coverage of the cardinal area is variable. In A. 

 floridana, the anterior portion of the cardinal area is often completely bare 

 (PI. 8, fig. 2) in some specimens, partly covered in others. The Pliocene 

 A. lienosa (Say) has strong groovings over the whole area, except for a 

 small bare space along the dorsal margin. 



