Agnostus was a blind, simply shaped trilobite reaching the size of a little fingernail. The cephalon, or head end, lacked the usual grooves or sutures that separate its central glabella from the side-cheeks. The pygidium, or tail, was rounded and smooth. In addition, Agnostus had only two segments in its thorax. This was the result of a pattern of thoracic segment reduction that began in the Early Cambrian and reached its peak at the end of that period.