266 AXEL A. OLSSON 



The shell is fairly large, oblong-ovate, thin or of a medium thickness. 

 Surface marked with even, concentric undulations on the umbones (spaced 

 about 3.5 mm. apart) but become irregular ventrally or replaced entirely 

 by growth lines near the basal margin. The general color is a dirty white. 



An average specimen measures: length 76.9 mm., height 59.8 mm., 

 diameter 22.2 mm. (a right valve, Pedernales, Ecuador), Fragments show 

 that the shell may grow considerably larger. 



Range — Gulf of California to Ecuador. Panama: Bucaro; Guanico. 



Ecuador: Limones; Pedernales. 



Subfamily MEKETRICINAE Fischer. 1887 

 Type genus Meretrix Lamarck, 1799. 



Shell trigonal, subequilateral, the left valve has an anterior lateral 

 tooth which fits into a corresponding socket in the right. The cardinal teeth 

 are arranged radially and are sometimes divided into smaller subsidiary 

 teeth as in Tivela. 



Genus TIVELA Link, 1807 



Type species by subsequent designation, Dall, 1902, Vemis corbiada 

 Gmel'in (:=Venus mactroides Born). 



Shell with a trigonal and often Mactra-Yxke. shape, the umbones being 

 placed near the middle axis and with close, adjacent beaks. The valves are 

 slightly to strongly convex, sometimes flattened, the surface smooth except 

 for the usual lines of growth. Hinge plate massive, generally with three 

 primary cardinal teeth in each valve, supplemented by several, smaller, 

 auxiliary teeth, the posterior cardinal tooth usually large, rugose or divided 

 into smaller segments. There is a large, anterior lateral tooth in the left 

 valve and a socket for it in the right valve. The ligament is attached to a 

 strong, nymphal plate. Adductor scars small and nearly equal in size. 

 Lunule large, setoff by a faint line or by a change of color; there is no 

 escutcheon. Color white or in varying shades of brown, often with a pattern 

 of radial rays and zigzag lines. The ventral margins of the valve smooth, 

 closing tightly or with a small, open, posterior gap in some species. 



Subgenus TIVELA s.s. 



Shell trigonal, mactroid, convex, with full, prominent umbones. Lunule 

 cordate, impressed, generally circumscribed by an incised or colored line, 

 its margin straight. Anterior lateral tooth large. Surface usually marked 

 with brown rays, the periostracum brown, rather coarse and pilose when 

 fresh. 



Subgenus PLANITIVELA, new subgenus 



Type species, Tivela plantdata (Broderip and Sowerby). 



Shell trigonal, depressed, or mildly convex, the umbones low, flattened. 

 Lunule elliptical, flattened, not strongly impressed. Nymphal area forms 

 a high plate separated by a slit or deep groove from an expanded or raised 

 margin. Coloration in rays or zigzag bands, the periostracum thin, vernicous, 

 and dehiscent. 



Subgenus PACHYDESMA Conrad, 1854 



Type species, Donax sttdtorv/nt Mawe. 



Shell of large or medium size, and often solid, elongate-trigonal. Hinge 

 with four cardinal teeth in each valve in addition to a strong, anterior 



