Cone shaped thing in matrix from Seymour Island, Antarctica

[David Cothran]

Near the K-T boundary there; below, I think.  A solitary rugose coral?  But the striations look wrong to me.  Loose on surface.

[historianmichael]

It looks like a piece of a Pinna clam partially covered in a concretion

[David Cothran]

Thanks, @historianmichael.  That looks right.  Much appreciated!

[abyssunder]

It could be a Pinnidae bivalve, but  could it be a rudist bivalve?

One picture is not enogh for the answer, in my thinking.