Trigonoceras, type genus, named by Hyatt, 1844, is unique among the Trigonoceratidae in the Treatise. It is the only genus included that has a subtriangular whorl section consisting of a broad concave venter and narrow dorsum. The remaining can be divided on the basis of shell morphology into those that are smooth, at least on the outer whorl, those with numerous equal longitudinal ribs or lirae, and those with prominent longitudinal ridges separated by wide grooves or sulci.(ibid Kummel)
