82 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
posterior small, semi-lunar; pallial impression marginal. Siphons long 
and slender, separate. (Tryon. S. S. Conch.) 
Type. Periploma incequivalvis Schumacher. 
Distribution. United States, West Indies, Panama, South America. 
Periploma discus Stearns, 1890. 
Plate 43, figs. 1, 3. 
Proc. U. S. N. M. f 13:222; pi. 16, figs. 1, 2. 
Shell thin, fragile, white, translucent, semi-nacreous; inequilateral, 
nearly circular, being posteriorly sub-angulated and flexuously squarish 
and produced; inequivalve, the left valve being more ventricose than the 
right; valves somewhat gaping; pallial impression narrow, shiny, distinct; 
sinus rather deep and rounded interiorly and curving up to the adductor 
scar; beaks small, nearly central, fissured; hinge a hollowed, spoon¬ 
shaped process (projecting inward from below the beaks), which holds 
the cartilage; this spoon-shaped cartilage cup or process is strengthened 
by an elongated callus slanting anteriorly; the exterior surface of the 
valves is finely wrinkled and linearly scabrous (more easily seen by 
holding a valve up to the light) and otherwise marked by concentric 
lines and zones of growth. Length, 41; height, 36; diameter, 14.50 mm. 
(R. E. C. Stearns.) 
Type in U. S. N. M., No. 105391. Type locality, Long Beach, Cali¬ 
fornia. 
Range. San Pedro to San Diego, California. 
Periploma planiuscula Sowerby, 1834. 
Plate 22, fig. 1. 
Proc. Zool. Soc ., p. 87, 1834. Jour. Acad. Nat. Sci. Phita 7; pi. 18, fig. 8. 
Per testa oblonga, planiusculta, inequivalvi, albicante, impolita, tenuis- 
cula; latere antico brevi, subrugoso; marginibus, antica subdeclivi sub- 
truncata dorsali rectiuscula; epidermide tenui, pallescente. Long., 2.4; 
lat., 0.8; alt., 1.8 poll. (Sowerby.) 
Shell of medium size, elliptical, inequilateral, inequivalve, fragile; 
umbos small, anterior to center; posterior extremity long and evenly 
rounded, but produced farthest a little above the middle; anterior portion 
much shorter than posterior, faintly biangulated; surface sculptured by 
fine concentric, incremental lines; hinge a hollow, spoon-shaped process, 
projecting inward from below the umbos; this cartilage process is 
strengthened by an elongated callus slanting anteriorly; pallial sinus short, 
cuneiform. (Arnold.) 
Type in British Museum. Type locality, St. Elena, Ecuador. 
