OF THE UNITED STATES. 51 
Var. A. Pl. ix, fig. 6. Smooth, expanded, compressed ; with 
three or four strong marginal plications. 
This shell in outline resembles the former, but is in 
other respects so different, that Iam almost disposed to 
consider it specifically distinct 5 should further compari- 
sons prove it to be so, I propose the name of O. nasuta. 
Var. B. PI. ix, fig. 7. Smooth, contracted; upper valve 
convex ; lower valve flat; with about seven strong marginal 
plications. Inner margin crenulated. 
So different in its characters from both the preceding 
shells, that were it not found with them I should not he- 
sitate to consider it a distinct species: should it hereafter 
prove to be so, it may be designated O. mesentferica. 
My specimens were chiefly obtained in the arenaceous 
marls near Shrewsbury, New Jersey. 
2. O. plumosa, (S. G. M.) PI. iii, fig. 9. 
Specific character. Ovato-triangular, thin ; lower valve con- 
vex, crenated near the hinge; dorsum marked with delicate 
strize, radiating in fasciculi from the beak to the margin. 
Length, about an inch and a half. 
From the blue marl of Arneytown, New Jersey. 
I possess a single valve of an old specimen, in which 
the striz are nearly obsolete. 
3. O. panda, (S.G.M.) PI. iii, fig. 6, and pl. xix, 
fig. 10. 
Specific character. Surface irregularly costate or undulated, 
plaited at the margin; with an irregular gibbosity, sometimes 
at the side, in others in the centre of the shell. 
The gibbosity of this species is very remarkable: 
