104 AXEL A. OLSSON 



Recent species appear to belong to three generic groups distinguished 

 as follows: 



I. Shell subovate, subcircular or slightly subelliptical, nearly equilateral, 

 the anterior side more evenly rounded, the posterior often somewhat 

 narrower and angulated at the end. Beaks nearly medial above the 

 cardinal area and but slightly opisthogyrous. 



A. Surface with relatively weak sculpture, the riblets low, often indistinct, 

 smoothish, or if radial riblets are present, they are usually overrun by 

 fine longitudinal threads or striations. Periostracum coarse and heavy, 

 dark in color and densely pilose. 



Genus Glycymeris 



B. Sculpture more strongly emphasized, the ribs well formed, smooth or 

 concentrically wrinkled but without superimposed radial threads or 

 striations. Periostracum weakly developed or v/anting. 



Genus Tuceton^ 



II. Shell decidedly inequilateral, the anterior side being the longer, well 

 rounded, the posterior side flattened or impressed. Beaks strongly 

 opisthogryrous, placed near the anterior end of the cardinal area. 



Genus Axinactis 



General Key to Panamic and Peruvian species 



I. Surface with relatively weak sculpture formed by low, inconspicuous 

 ribs, generally overrun by threadlike subsidiaries or the surface may 

 be nearly smooth. 



Glycymeris 



A. Shell small or of medium size (seldom above 45 mm.), ovate-sub- 

 circular in shape. Cardinal area small, short, triangular. 



1. Shell ovate in shape, the height often appearing slightly greater, sur- 

 face generally appearing almost smooth except for underlying sub- 

 obsolete radial ribs, the coloration brownish or cream, plain or lightly 

 rayed, the interior white. Peruvian. 



G. ovata 



B. The shell larger and the sculpture generally more pronounced. Panamic. 

 Ba. Shell relatively large, subcircular in shape, nearly equilateral, the pos- 

 terior side differing only slightly in shape from the anterior. 



2. Shell large (up to about 100 mm.), the surface beneath the coarse 

 periostracum finely marked with transverse wavy or zigzag lines, 

 especially over the umbones. Gulf of California. 



G. gigantea 



3. Shell smaller, the surface marked more densely with brown spots or 

 streaks. 



G. macidata 



Bb. Shell of medium size, subelliptical in shape, inequilateral, the posterior 

 side somewhat longer and produced. 



4. Surface finely sculptured by low ribs overrun by linen-like subsidiaries. 

 Color more uniformly brown or cream; spots if developed are more 

 sparingly distributed. 



G. lintea 



