Trinodus is a very small to small (about 1 centimetre or 0.39 inches) blind trilobite, a well known group of extinct marine arthropods, which lived during the Ordovician (Tremadocian to early Hirnantian),[1] in what are now the Yukon Territories, Virginia, Italy, Czech Republic, Poland, Denmark, Sweden, Svalbard, Ireland, Scotland, Wales, Iran, Kazakhstan and China.

Arthrorhachis is derived from the Greek ἄρθρον (árthron, "joint") and ῥάχις (rháchis, meaning axis, spine, ridge or backbone).

Although this material is distorted or incompletely preserved, it is very similar to the pygidium of Arthrorhachis tarda.

Species with a rear rhachis lobe longer than the postaxial region are henceforth combined in Geragnostus, all others are assigned to Trinodus.

[2] Trinodus elspethi, which - as an agnostoid - only has two thorax segments, has at least nine larval stages (or instars), three meraspid and six holaspid, in its life.