42 Say on Shells, &e. 
markable species; from which, however, it is sufficiently 
distinct by the much developed form of the lobes. 
In these specimens a considerable portion of the shel! 
remains exhibiting its beautiful iridescent colours. 
Mr. Nuttall gave me the followimg account of this species. 
It occurs in the ancient alluvium of the Missouri, or clay 
formation, reposing adventitiously on the chalk stratum of 
this region, and imbedded in the indurated shistose beds, 
amidst other shells, and in the beds which overlie more or 
less intimately the Xylanthrax or Surturbrand; they are 
gradually and regularly acuminated through a length of 
twelve or eighteen inches, being from three to four inches 
broad at the base and diminishing to less than half an inch, 
but a perfect apex or base has not yet been discovered.— 
They are of frequent occurrence, washed out on the banks 
of the river, from White river of the Missouri to the Man- 
dans, but at the same time, locally and not uniformly dis- 
tributed. 
Genus Ostrea. 
O. convera, Oval, inequivalve ; inferior valve remarkably 
convex, with a longitudial indented line on one side, slight . 
ly auriculated, or rather, angulated each side of the hinge, 
a longitudinal, transversely wrinkled depression, each side 
before the hinge ; ligament cavity oval, placed beneath the 
apex ; supertor valve suborbicular, flat or somewhat con- 
cave, radicated from the apex to the periphery, annual in- 
crements strongly marked; hinge each side before with 
transverse ruge. 
Length of the convex valve nearly three inches, breadth 
‘wo and an half—depth about two inches. Cabinet of the 
Acad. Nat. Sciences. 
A perfect specimen was found by Mr. 8. Wetherill near 
Burlington, N. J. Thave since obtained a ferrugmated one 
at Mulliger Hill in the same state. Itis remarkable for the 
great convexity of one of its valves and by the angles each 
side of the hinge.—It closely approaches to the genus Gry- 
“phea ; the lower valve is even proportionably more convex 
than that of 4nomia gryphea, and is also furnished with the 
indented line or lateral lobe as in that shell, but the umbo 1s 
not prominent, the superior valve is as operculiform as that 
