Trimeroceras is a genus of straight oncocerid (Nautilodea, Cephalopoda) from the Silurian (Telychian[2] to Ludlow) of Europe, China, and North America.

Trimeroceras bulbosum and T. cylindricum are found across Eurasia,[3] while T. ellipticum is restricted to Scotland and Wales,[2] and T. gilberti is only known from two specimens from Indiana.

[1] Like other related oncocerids, the shell of Trimeroceras is straight and of equal width and height up to the septum of truncation and bearing horizontal markings, with a later curved apical part.

[1] Trimeroceras had a restricted slit-like aperture which, on top of the hyponomic sinus acting as water-jet funnel, allowed only a small number of tentacles to exist which would have been much smaller than that of modern Nautilus.

[1] Based on the structure of Trimeroceras' connecting rings and the mass distribution of mature oncocerid shells in carbonate facies, it is possible that they used modified body chambers to brood eggs internally before spawning in shallow water environments.