Nassauoceras is a tetragonoceratid

Tetragonoceratidae

The Tetragonoceratidae is a small family of nautilitids constituting a part of the superfamily Tainocerataceae in which shells are coiled with a generally quadrate whorl section. Coiling is either gyroconic or evoluute with a slight dorsal impression. Flanks diverge from the umbilical to the...

 nautiloid from the Middle Devonian of Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, the shell of which is evolute with a wide, deep umbilucus, slight dorsal impression, low arched venter, rounded ventral shoulders, and flanks that converge dorsally so as to produce a subtrigonal whorl section. Nodes are present on the ventral shoulders. Sutures have shallow ventral and lateral lobes. The siphuncle is near the ventral margin.

Nassauoceras differs from Tetragonoceras

Tetragonoceras

Tetragonoceras is an extinct prehistoric nautiloid genus from the nautilid family Tetragonoceratidae that lived during the Middle Devonian, found in Canada....

in that the latter is gyroconic with a subquadrate whorl section and angular ventral shoulders, but is somewhat similar to Wellsoceras

Wellsoceras

Wellsoceras is a tetragonoceratid that starts off with an evolute shell in which whorls are in contact, but has a mature living chamber that diverges and becomes free. The whorl section is slightly wider than high and is faintly subquadrangular. Flanks are slightly convex and tend to comverge...

in which whorls are in contact in the early growth stages and shoulders are also rounded.