Tainonautilus has an evolute shell with a subquadrate whorl section.
Differences lie primarily in the symmetry of the whorl section and in the suture.
The whorl section is much higher than wide with the maximum width slightly ventral of the umbilical shoulders.
The whorl section is compressed with straight sides and strongly arched venter.
The whorl section is subquadrate and compressed, so as to be narrower than high.
The whorl section is typically inflated or depressed.
Members of the Trocholitidae vary in whorl section and siphuncle position.
The whorl section is slightly wider than high and is faintly subquadrangular.
The earlier, related Domatoceras is evolute, with a more quadrate whorl section.
The whorl section of Chouteauceras is ovate, higher than wide.
