74 AXEL A. OLSSON 



passing through a gap along the middle of the ventral margin. Valves often 

 distorted and irregular in shape. Sculpture more or less decussated, formed 

 by small or medium-sized radial riblets and concentric threads. Ventral 

 margins of the shell smooth, not crenulated or fluted by the ribbed sculp- 

 ture. 



Subfamily ANADARINAE Reinhart, 1935 



Shell mostly free, the byssus small, the margins of the valves fitting 

 tightly. Beaks are generally prosogyrous. Sculpture typically formed by 

 strong, coarse, radial ribs, smooth or noded which form strong flutings 

 along the ventral margins. 



Subfamily NOETINAE Stewart, 1930 



Shell mostly free, with or without a byssal gap. Beaks opisthogyrous. 

 Cardinal and ligamental area wholly or mostly anterior of the beaks, and 

 generally marked with vertical grooves. Sculpture of fine or strong, often 

 flattened ribs which flute or crenulate the margins. Posterior adductor scar 

 bordered by a flange. 



Subfamily STRIARCmAE 7 



KEY TO THE GENERA OF ARCIDAE IN THE PANAMIC-PAFICIC REGION 



I. Arcinae. Nestlers and borers, the shape of the shell regular or dis- 

 torted, the ventral margin with a byssal gap placed a little anterior 

 of the middle. Surface sculpture composed mostly of fine radial rib- 

 lets or threads, noded or decussated by concentrics. Inner, ventral 

 margins of the valve smooth, little affected by the external ribbed 

 sculpture. 



A. The ligament covers the whole of the cardinal area, usually on both 

 sides of the beak. 



a. The major part of the ligamental coverage lies behind the beaks; 

 the posterior section of the hinge line the longest. 



1. Shell rectangular, often expanded or winged posteriorly and with a 

 prominent umbonal angle. Sculpture formed by numerous, somewhat 

 irregular riblets. Cardinal area is relatively high and wide, the 

 ligamental grooves forming tent-shaped incisions, usually few in 

 number and irregular in their development. Color brown, often with 

 zigzagged markings. 



Genus Area 



2. Shell not conspicuously winged or emarginated and the umbonal 

 angle rounded. Sculpture finely ribbed and decussated. Cardinal area 

 elongated, thickly covered by tent-shaped, ligamental grooves. 



Genus Barbatia 



b. The major part of the ligament lies in front or anterior of the beaks, 

 the anterior section of the hinge also much longer. 



■^ A fourth subfamily, the Striarcinae may be recognized and would include the 

 genus Arcopsis. Most genera of the Striarcinae are fossil. They resemble the Arcinae 

 by their external sculpture and in having a ventral byssus, sometimes resulting in a 

 gap, but they have a vertically striated ligament and the raised or flanged adductor 

 scars of the Noetinae. 



