The Agnostida appeared by the end of the third stage of the Cambrian, after the Redlichida and at roughly the same time as the Pytchopariida. This order survived to the end of the Ordovician although its members were far more common during the Cambrian.
Trilobites in the Agnostida don't look like any other type of trilobite; this order is easily distinguished from the others. Trilobites in the Agnostida are small, typically 4 - 10 mm long. The head has a parabolic outline and is the same size and shape as the pygidium. Except in the superfamily Condylopygoidea, the glabella is widest at the back of the head and narrows towards the front.
There are usually only two thoracic segments, but some members of the suborder Eodiscina have three.
Most members of the order are eyeless.