360 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvi. 



Genus VENUS (Linnaeus) Lamarck, 1799. 



Type, Venus mercenaria Linn^us. 



Siiell largo, heavy, earthy, trigonal; with faint radial and stronger 

 concentric lamellar sculpture; lunule and escutcheon well defined; 

 internal margins crenulate; pallial sinus small, triangular; there are 

 two bifid cardinals in the left valve, one bifid and two anterior simple 

 cardinals in the right valve, with a rugose area in each valve repre- 

 senting a supplementary cardinal below the ligament, the rugosities 

 interlocking when the valves close; the ligament is strong and wholly 

 exposed; the posterior dorsal margin of the right valve grooved to 

 receive the edge of the left valve. The genus is American. 



It is Mercenaria Schumacher, 1817, and Orassivenus Perkins, 1869. 



Genus MARCIA (H. and A. Adams, 18B7) Fischer, 1887. 



Type Venus exalhida Dillwyn. 



Shell large, subquadrate, concentrically lamellose and striated, with- 

 out radial sculpture, and with a dull, earthy surface; internal margins 

 smooth; pallial sinus small, angular, free; hinge with three left and 

 four right cardinals, the middle ones larger and grooved above. Aus- 

 tralasia and southern South America. 



Subgenus Marcia s. s. (See above.) 



This is a Venus without hinge rugosities, radial sculpture, or mar- 

 ginal crenation. There is a well-defined lunule, but no defined 

 escutcheon; the ligament is exposed. 



It is Katelysia (part) Romer, 186T, not of Tryon, 1881. 



Subgenus Katelysia (Romer, 1857) Tryon, 1881. Type, Vmus sca- 

 larina Lamarck. 



Shell rounded-trigonal, subcompressed, very inequilateral, sculp- 

 tured with concentric riblike ridges, sharper distally, polished, por- 

 cellanous, with no radial sculpture; coloration lively, anterior end 

 sharper; lunule smooth, circumscribed, escutcheon defined only by 

 absence of sculpture; ligament short, internal margins smooth; hinge 

 plate buttressed between the pedal and adductor scars; three cardinals 

 in each valve, the anterior right and posterior left slender, entire, the 

 others grooved or bifid; the anterior left and posterior right dorsal 

 margins beyond the hinge plate sharply grooved to receive the beveled 

 edge of the opposite valve. South Seas. 



Section Katelyslri s. s. (See above.) 



The inequilateral ovate form of these shells is quite striking. 

 Chamelea Chenu, 1862, not Morch, 1853; Murcla (part) Romer, 1857, 

 not Koch, 1885; and Cafdysta Fischer, 1887, are synonymous. 



Section Ileiiiltapcs Romer, 1857. Type, Ve7ms rimularls Lamarck. 



