Eutrephoceras carolinensis
Middle Eocene
Castle Hayne Formation
Rocky Point, Pender County, NC, USA
Fine 30mm nautiloid on a small piece of matrix. RARE

Eutrephoceras lived across the whole Cretaceous Period and survived past the K/T extinction.  Miocene specimens have been reported.  The specimens from the Castle Hayne Formation are rather fragile and tend to be found in pieces.  Here’s one in great shape.

Eutrephoceras typically possess nearly globular conchs (shells).
The whorls are reniform (kidney-shaped) in cross section and broadly rounded on the sides and lower edge.
On the upper edge it is only slightly curved. The surface of the shell is usually smooth, but can sometimes be sculptured. The suture patterns are slightly sinuous, though it can be more or less straight in some species. The umbilicus is small and barely noticeable, sometimes hidden altogether.
The septa are averagely convex towards the tip.