The genus Dimorphoceras was instituted by Hyatt, the genotype being Goniatites gilbertsoni (Phillips). The shells are involute and compressed, having the umbilicus very small or closed. The test shows fine lines of growth and a deep hyponomic sinus. Dimorphoceras is differentiated from other goniatites by having either one or both of the sutural lobes sub-divided by a small subsidiary saddle. Specimens are not generally abundant; and owing to the feeble ornamentation of the test, distinction between species is impossible unless uncrushed specimens showing sutures are obtained.