Trimerus is one of North America's largest trilobites, reaching over 20 cm (7.9 in) in length.

It had a thorax composed of 13 segments with weak trilobation, a large subtriangular head terminating in an expanded rostral plate, a two-pronged hypostome, and a triangular pygidium.

Its tiny compound eyes and the shovel-like anterior of the head suggests a burrowing lifestyle, and an exoskeleton marked with many small pores which, in life, probably housed hair-like sensory setae in life, allowed the trilobite to feel which portions of its body were covered with sediment.

[2] A number of species previously assigned to other genera were included, and a number of species previously assigned to Trimerus were excluded, variously reassigned to the trilobite genera Dipleura (Dipleura dekayi has been frequently included within Trimerus), Digonus, Burmeisteria, and Wenndorfia.

Trimerus (Edgillia) This group is known from the Late Silurian to the Early Devonian.