Eutrephoceras "elegans" var. nebrascensis is described as having a subglobose (not quite rounded) shell which is broadly round on the flanks (sides) and venter (outside edge). The umbilicus is closed in young to medium sized specimens and involute (only slightly open) in larger specimens. The size of the whorl (a 360 deg. rotation of the shell) increases rapidly with each rotation. with the height and width ratio at 3.5 to 3.0 The sutures (or septum walls) have a straight curve forward near the umbilicus, and slightly backward on the side, then slightly forward again at the venter or outside edge. The shell surface is smooth with distinct growth lines and faint longitudinal costae (ribs). The siphuncle placement is about 1/3rd the distance in from the outside edge. Eutrephoceras nebrascensis has been found along with Didymoceras cheyennense through Baculites eliasi Range Zones (74 - 71 million years ago) and was most prolific during the Baculite compressus and Baculite cuneatus Range Zones (73.5 - 73 million years ago) of the Western Interior of North America.
You can see the siphuncle tube connecting the chambers in the photo below.
Eutrepha = nourishing + ceras = horn.
Kingdom: Animalia, Phylum: Mollusca, Class: Cephalopoda, Order: Nautiloidea, Family: Nautilidae, Genus: Eutrephoceras, Species: nebrascensis.
Eutrephoceras nebrascensis 
Quantity in Basket: None
Code: FA-290
Price: $45.00
Shipping Weight: 0.84 pounds
Time: Upper Campanian Stage, approximately 73-71 million years ago.
Location: Pierre Shale, South Dakota, USA
Dimensions: 3" x 2-1/2" x 2-1/4"