OF THE UNITED STATES. 51 



Var. A. PI. 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 I am almost disposed to 

 consider it specifically distinct ; should further compari- 

 sons prove it to be so, I propose the name of 0. 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 0. mesenterica. 



My specimens were chiefly obtained in the arenaceous 

 marls near Shrewsbury, New Jersey. 



2. 0. plumosa, (S. G. M.) PI. iii, ft%. 9. 



Specific character. Ovato-triangular, thin ; lower valve con- 

 vex, crenated near the hinge ; dorsum marked with delicate 

 striae, 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 striae are nearly obsolete. 



3. 0. panda, (S. G. M.) PI. iii, fig. 6, and pi. 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 : 



